You should actually develop a way to get parental consent instead of simply rejecting underage users. This is a blight that has deprived children from literally any and all online opportunities, for more than 20 years (!!!), and what's worse - people think it's "for the children's good". Often ignoring the fact that the very similar online dangers - inappropriate content or behaviour - can be found offline too: schools, playgrounds...
COPPA requires PARENTAL CONSENT. In addition, for basic mobile games, the data necessary to run them are quite minimal, so parental consent can be verified through email, just like on Roblox. Also in the age of ubiquitous freemium SaaS apps and services, we shouldn't be surprised about credit cards - if paying for a subscription is easy, so is paying to verify the identity if you're a parent. Stop pretending that if you keep kids out you keep them safe.