A child’s assertion of abuse, especially if the same thing has been reported by a few children, is not merely an allegation.
Depending on the circumstances (do they know each other or have someone in common to coach them), multiple accusations might very well be raglayim l’davar.
I’m not sure what your question to me is – whether or not something is raglayim l’davar is obviously a halachic shaila and must be asked to a posek. Otherwise, no matter what the percentage of false accusations is, there would certainly be many more if every child was always believed with no other evidence.
You analogy to Leiby Kletzky a”h is beyond me.