FWIW, I've tried to do that for my almost-5-yo by starting with an account for him only: unfortunately, even with family-controlled account (whatever their terminology is), I wasn't able to do that (at least not with a non-gmail address). Google kept throwing weird errors after attempting to validate the account with 2FA through SMS on my phone number. Yes, I even tried "family link" or whatever using an Android phone (I originally tried using only a computer).

It seems it might be related to legislation surrounding account ownership by minors, but I have no idea how to confirm it, esp since it's supposed to work when a parent controls the account.