Well....it all depends on how much crystals you have to spend on average to get a new child.
From my experience, to ensure a new child released, I need 40-60k usually.
Besides, my average numbers of getting a 5 stars from crystals pull is 100-120.
Even 9% pulls, there were 9 days that I didn’t get anything.
So yup, onyx is good , but most child I could get is already full lvl... or at least those common use already full lvl.
Some ppl view crystals more valuable than onyx, me as well.
If I am free on hot time weekend , I usually use 2 lvl60 child to carry 3 2stars child at stage 9-4-2, 90 staminas trans to 60 crystals and 6k golds. Just kinda troublesome to keep changing team.
But to newbie or who just really need onyx, definitely farming abyss is a great choice.