Quote from Phoebe in The One with Monica's Boots
Phoebe: Ross, if they don't get along, then you should smooth things over. Make them be friends. Ross: Phoebe, you can't force kids to be friends. Phoebe: Sure you can. Give them some blocks. Put them in a playpen. Ross: Playpen? Ben's 7. Phoebe: Your kid is 7? He's really small.