Manda, as it is a preschool, and doesn't actually cater for children under 3, technically they don't need a nappy change facility. This would also impact of staff ratios/wages/fees etc.
At our place, we prefer the children to be in knickers - pullups are just like nappies, and children begin to rely on this after a while. The staff shouldn't mind any little accidents - that is all a part of it. If it was me, I wouldn't be giving her any choice, knickers all the way - but perhaps I am a toughie! Just make sure you pack heaps of knicker snd clothes.
In our 3's, the children are asked often throughtout the day, and 'going' with a friend always helps lol