I assumed iOS had multiple users, no? Android has native multi-user functionality, primary, secondary, and guest users. It allows to do exactly what you are asking. Samsung has an additional feature where a folder keeps a unique set apps (sandbox) that can be completely different than the rest of the phone. Samsung calls it a secure folder. That allows each set of users to have two set of apps if needed.
my $200 Xiaomi phone has exactly this functionality - 'second space', unlockable by second fingerprint
It would be so fantastic if he could just start HIS apps and would access a restricted Prime account. My 6 yo is the same and since he was 2 1/2 I switched by iPhone twice. So I don't see the case that it's not important as they will get their own ones when they are 8 or so - that's 5 generations of iPhones.