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.