How do I use/transfer my kids iTune balance onto my family plan
There is not a transfer function since the value was spent when it was associated with an AppleID. 


Can I redeem all my iTunes gift cards at once?


If you queue up your purchases, remove the accounts with a credit from the family and then spend down the balance to zero, then join back in the family. As long as the purchases share with the family, it won’t matter that a non-primary purchaser purchased it. The main thing, is each account can spend down their balance if you let them purchase. 

Going forward, only redeem credits from the master account that carries the payment terms of you want to prevent or restrict purchases from other accounts in general.