Follow these limits if you want to be dead safe with your child accounts and want them to sustain longer :
Follow-unfollow : 15-24/Hour and 140-180/Day
Likes : 15-25/Hour and 150-200/Day
Comments : 4-6/Hour and 25-40/Day
Dm's : 4-8/Hour and 35-60/Day
These are basic limits which will keep your accounts safe, You can go above them yes but better not risk it and play the long term game

If your account is completely new, Start with half of the limits written above and gradually increase in a week.
Simply repost it from your main account, Or arrange a few posts like maybe around 6-12 for the childs and mass post it in all of them, Tried and tested and it actually works.
Yes, Depends on how you portray it, You can actually keep those child accounts like your other accounts or basically a fanpage too. Follow backs and conversion to the main would be much more when you keep the child accounts as if they belong to the main account, Tested and works