I am not sure about your first question. "Watoto wako = your children " this indeed is a statement. Did you mean to say a complete sentence? "The children are yours." will most probably translate to "Watoto ni wako" but this is just a guess. I may be wrong. Afterall, today is my third day learning Swahili (05 Mar 2017)
What do you mean by "what happened to" ? If you ask a bit more clearly I will be happy to try and answer.
I will try to guess what you meant and answer; :)
"wako" here means the 'owner' of "watoto" is singular
"wenu" will mean the 'owner' of "watoto" is plural .
I hope that helps