State that ... you really want to help the children to manage their bedwetting in exactly the same way that she manages it at home
Ah, one of those occasions when tone didn't come across on screen.
What I mean is that she cannot refuse to let you manage it as she does, so she has to tell you how she manages it. It is quite likely that that includes equipment you don't have - mattress and duvet protectors, alarms, lots of extra sheets - or procedures you can quite reasonably object to - allowing them to sleep in wet clothes and bedding all night.
Once she says what she does, then you can either object - 'No, I'm not leaving them in wet sheets all night' - or ask for all the equipment or all the money towards it. She will have LOTS of sheets, for example, so you could simply agree that you will bag them up for her to wash when she gets back.
Or it may be that they don't wet much at home - there were lots of 'little' ways I managed DS's bedwetting that became so ingrained that I didn't consciously think about them and by saying e.g. Dnephew has already wet 2x each night and DNiece once' you are showing the extent of the problem.
As I said before, I take lots of 9-10 year olds on residential stays each year. Almost all parents warn and equip us for bedwetting. The only time in all the years I have been irritated with a parent was when they had not warned us and not only was the child wet every night but also had no spare pyjamas or bedding. 'Oh, they only wet a couple of times a week at home, we thought since they weren't away for very long it probably wouldn't happen and we didn't want to make them feel different or embarrassed by mentioning it to you or by giving them their usual drynite pants'. Grrr!