I need help with multiplayer issues!!!!!

I recently got my little brother a minecraft account (java) and want to add hit to my realm but it says multiplayer is disabled and I have searched on all account settings (mojang microsoft xbox ect) that could be linked to it but I couldn't find it for the life of me. Any feedback would be great

U may have given the right day of birth... Give him an old fake one, it fixed the prblem with me. I think u have to be "older that 12" to turn multiplayer on...