I had this EXACT situation happen a couple of months ago! Dcg peed on the back of her pants and undies and had no undies in her bag, just pants, tops etc. My daughter is 1 year older and had a bunch of undies that no longer fit her. So I put one of those on dcg and told mom at pick up and gave her a few more pairs to keep that no longer fit my girl. The parents were totally fine with this and thanked me. I don't see what the big deal is, they were clean, un-stained and in excellent condition.
To each their own I suppose!