Always state which line is causing the error. You have 3 somewhat similar references (6 if you want to count them all). We're left guessing.
Maybe you left one subform control with the default name of "Child#" where # is some number. In other words, the syntax may be correct but the references are not. Often it's just an invalid spelling/reference?
EDIT - even if it works on another event, the above could still apply - if you retyped it and it is correct in the working procedure. If it errs on the first reference, rem it out and see what happens on subsequent lines. I agree that if a recordsource is altered, it doesn't seem to need a requery.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.