I absolutely love this game. It's so adorable, the UI is so clean, the gameplay loop is so simple yet so fun, it's wonderful!
I ran into a bug a couple times where cherubs would get caught in a never-ending loops of the watering animation, so they were taken completely out of commission for the rest of the game. That was the only real bug I found while playing though.
A couple things for user experience: It would be really great to have the cherub's names under them as little labels so I can tell them apart. A lot of times cherubs would spawn behind trees or in other spots that are kind of hard to click. If I spawn a tree in front of another tree it *really* messes things up, and while I quickly learned my lesson, it felt pretty bad to have happen. Also it would be great if the cherub menu closed automatically after I transform a cherub.
As far as design goes, I really like the idea of basic cherubs turning into other kinds of cherub, which turn into different trees that give different resources. That loop, in a vacuum, is golden. However, (since I have to give critique) I think there are a couple ways the game could be improved while staying at its current scope.
First is an easy one: the sound is wayyy too low. The first few times I played through, I didn't actually know there was music at all because my volume wasn't turned all the way up.
Second this game is kind of "all carrot, no stick". The player is motivated to take care of the cherubs because they turn into different forms and will eventually spawn more resources. The transformation aspect is inherently cool, and has historically been an extremely effective motivator in video games. Gamers pretty much universally love when things transform, and once they know it's possible they'll go to the ends of the earth to make it happen. I think that even if the cherub's *didn't* eventually give the player more resources, I'd still be motivated to make them level up (and I think a lot of other people would be too), but the resource-gain is definitely the cherry on top (although it does add a weird level of depersonalization where this active, living creature you've been caring for is turned into an inanimate thing with no reference to the creature it used to be, but I think that could probably just be solved by keeping the cherub's nameplate under the tree so that players can like, look at a fountain and go "ah, yes, that fountain was once Jeremy. Jeremy is still with me, in a way. I love Jeremy")
The "no-stick" side of things come with the fact that there aren't any negative consequences for the player if they don't do their tasks/don't do them well enough. I think the best virtual pets have an element of tension to them where you don't want anything bad to happen to this little thing you're taking care of, but this game doesn't have that element. When basic cherubs starve, they turn into evil cherubs, and when non-basic cherubs starve, nothing happens. I don't think it would be too difficult to implement a way for cherubs to die (or, if that's too morbid, run away), and it would be an extra incentive for the player to make sure they're taking good care of their pets.
I also think that sleep is kind of superfluous. I understand the instinct to put it there, since resting is a mechanic in pretty much every virtual pet, but in this game it really doesn't do anything other than make you less able to interact with the cherub. There were times when a bunch of my cherubs went to sleep at the same time and I was left twiddling my thumbs and gathering resources, unable to interact with my beautiful pets. You could rework sleep if you really want to keep it, but I don't actually think it's a necessary thing to have in the game, especially since these creatures are somewhat mystical/magical/supernatural, so you could get away with saying that they just don't need to sleep.
Another small thing is that, after cherubs level up (with the notable exception of the evil cherub), their stat meters don't go down by very much (other than the water cherub, but the water cherub likes water so much that its meter gets filled up super quickly), so it only takes a couple plums/flowers to get their meter high enough to transform. This makes it feel like the cherub might as well go straight from basic to final form, since such minimal effort is required to move on from stage 2 to stage 3. I think a good way to fix this could be by leaning into cooldowns for each action, like you do with the pet button. It would make sense logically for a cherub to not want to do the same thing again immediately after, and it would also keep players from hyperfocusing on a single cherub, and encourage them to instead care for each cherub a little at a time.
I know that was a lot of critique but it's only because I really, really dig this game. As a final note, I want to talk about some things that you could focus on if you want to keep working on this game. Like I said before, the formula itself is golden, so don't change anything about the core loop. The obvious way to expand this game would probably be just to add more stats with their own resources and cherub types, but it would be pretty unhelpful of me to just suggest "add more stuff". Instead, I'll talk about some things that you already do really well that you might want to look into expanding.
I really like the fact that some types of cherubs like certain things more than others, and that's definitely something worth exploring. Also, I know I mentioned the evil cherubs in the critique but I really, really like the fact that there's a transformation that occurs when a special condition is reached outside of the normal "fill the specific stat bar" requirement. I didn't actually read the "try starving one, for science" thing on the tutorial menu, so when I accidentally neglected one of my cherubs and it transformed I was surprised and totally elated. I think that having more specific transformations that trigger with secret conditions would be a more interesting way to add more types of cherubs than just adding more stats.
One final thing I'll say is that the only thing gamers like more than transforming things is combining things. I'm not saying add a breeding ground or fusion station or whatever necessarily, I think that wouldn't quite fit with the vibe of the rest of the game. I'm more saying like, what would happen if someone maxed out a water cherub's food instead of their water? What would that turn into, what ratio of resources would it give? Stuff to think about, I guess.
Sorry I wrote a whole manifesto in your comments section, I tend to do that when I really really like a game!