Jalan

Since you mention a child, note that each immigrant needs their own petition.  Assuming the child custody agreement is part of a divorce decree, of course the whole decree needs a certified translation to English.  If the custody agreement is separate, it isn't needed until the interview, where it would not need translation as long as it is in the local language where the interview takes place.  For other than the visa process, it may be a good idea to get it translated though.

Be very sure what you have in this regard, because if the other parent is Ukrainian, there is no guarantee they will ever see their child again.
 
For immigrating to the USA, there must be either a court order of full custody, or permission from the other parent to immigrate permanently to the USA.

With China as an exception, "Translators" are not certified.  "Translations" are certified by the translator.  The translator certifies they are fluent in both languages and that the translations a complete and accurate.
 
Correct (again with China as an exception) no notaries need be involved and no apostille for any documents.

if u are the  beneficary and children are overseas, each child needs his//her own 
if translations are done by a certified translator ,  no need for notary
no apostle needed for any of the documents 

Which process are you going through? DCF? Inside US or submitting I130 to USCIS for consulate process? 
No apostle needed for birth certificates. 
child custody agreement should be translated into English. 
some say you can do it yourself but others say it shouldn’t be the petitioner or the beneficiary translating documents. 

I would estimate 75% or more U4U recipients are outside Ukraine.  6 people I met sponsored by my wife's friends were residing in Poland and Germany.

U4U is much much quicker and then adjust.

Where did I say that, where did I even imply that.

k3 is obsolete