Hang on a minute!!
Im a NNEB and worked in nurseries and pre schools - there could be any number of things gone on,
Working on the assumtion dd had wet herself,perhaps it was a little wee from holding on?
perhaps she did it and didnt tell anyone?
perhaps the assistant was the only one in the room and there was no one there to relieve her whilst she took dd to the bathroom?
Perhaps she knows you ex partner is having a bad time and wanted to rectify the situation asap so as not to distress her?
perhaps the assistant felt awkward with you there?
maybe
your dd had spilt her drink/the water/sat in something?
Its very hard to think that a child has done a full wee and not left a puddle that would definitely be discovered within minutes.
Of course these things do happen and whilst I am secure knowing all my charges were well looked after I cant obviously say all other childcarers would be the same,I dont think this was in any way neglectful or lazy.
Personally Id let it ride this time but perhaps speak to the assistant tomorrow.
Btw if a previously dry child wets herself it can be an indicator of a urinary tract infection.