I would talk to mum and rather than ask, insist that she not send child in dresses as you have seen a definite increase in accidents as it is harder for the child to toilet herself. Asking implies that the parent has an option when in reality the dresses do not work for daycare and are inappropriate clothing for the reasons you have given and are not acceptable. The other thing you could do is just take the tights off and then toileting should be relatively easy as long as the girl can scoop her whole dress up. That's the other problem with toileting with a dress on when it gets in the potty or toilet.