Answered by Sidi Mostafa Azzam
Is it permissible to carry a child who might have a wet diaper in prayer?
As The Immaculate Raiment says, “The following must be free of unexcused filth: your body, what you are supporting, and everything touching or connected to them.” If you carry a child that is filthified with urine or feces, what you are supporting is not free of filth, so your prayer is not valid. But that is in the case that you are, in fact, supporting filth. But what if last you knew, the child was pure, and now she may be filthified? Does that invalidate your prayer? No. Your certainty that the child was pure is not negated by your uncertainty about it being filthified, and so the child is still considered pure. As the fiqhi principle mentions, certainty is not removed by uncertainty. The upshot is that if you are certain that the child has been filthified, your prayer is invalid; if you are not certain that the child has been filthified, your prayer is valid.