I understand both sides but I'm gonna have to side on the mother on this one. Ain is just a child and children are suppose to be safe and free from harm regardless of what they'll become. I do understand the father's side though but I suggest he might want to postpone his poison education (though I don't believe taking in posion would make you a "strong" successor) .
Just stating my opinion. Cheers (●'◡'●)ノ
I think they should have the poison when younger that way it’s easier to develop the immune system for this time. That way when he has his debuts into royal society he’s ready. I get he should have a childhood and it’s good that the FL is giving that for him but she needs to be more aware of the circumstances. Plus their families role seems serious and not to be taken lightly especially with Ain being the only heir. If Ain gets poisoned and gets sick that’s when their enemies can strike.
Clearly, he's hungry for the other guy (▰˘◡˘▰)