Does anyone know how to get some of the past childs, or the special event ones?
Colab units are only obtainable during the colab events. If you have a copy of a colab unit you can get more by using HoR. Any other unit can be summoned thru the normal summon pools. If you need 5* units i would suggest summoning on the 6% banner and pushing events to "purchase" the new units with event currency when possible.
I was aiming for the Blaze Blue Child's. Thank you for the information very helpful