Try making an entirely new child BP from the working parent and see if this does not resolve the issue.
If not, in your Child AnimBlueprints, are you using animation overrides? If so, are you using animations from AnimSets that are not yet compatible with UE4.15 like from the Movement Animset Pro?
Also, if you are using animations from the Animation Starter Pack, you may want to add and replace the current animations with the latest versions. To do this, move current animations to a folder marked “x.” Add the Animation Starter Pack to your project per the usual, default location. Delete the animations you are currently using and, when prompted, replace with the ones you’ve just imported.
Let me know if this does not resolve the issue.