Could your dp write to his ex and cc the head teacher and anybody else as appropriate to say that he does not want the children to be removed from school during term time, that he is willing for them to stay with him for the duration of ex's holiday and that he will be responsible for getting them to school during that time and that ex will be liable for all fines that he incurs as a result of taking them away on holiday against his wishes - and that if she doesn't pay them and he ends up having to, then unless she arranges to pay him back separately, he will take this as notice that he will withold it from maintenance payments. Furthermore, he does not know how or if he is able to stop his wife from taking his dc on holiday as he does not want them to go during term time but that he would like it to be put on the record that he doesn't want his dc to go in this instance. and that he is taking steps to find out if there is anything he can do to this end.
Hopefully this will help to cover his back should he get taken to court along with his ex and maybe if she sees it written out and that the head teacher has been involved, maybe it will make her think twice, especially if she sees that by taking the dc away without his consent she will be accepting liability for any fines incurred by your dh.
The head teacher may also be able to advise on if there is anything that can be done to stop the kids from being taken.
Make sure they get sent by recorded signed-for delivery.
It's all very well saying that the dp shouldn't withold the money from maintenance but if the ex is doing this against the wishes of the dp and it could have a financial impact on the dp, then quite frankly, as he knows the kids won't starve, I don't see why he should make her liable for any costs he incurs as a result of her actions.
He's not refusing to pay for a school trip or ballet lessons or a laptop or winter coat - each of which would be discussed in terms of affordability and need vs want, as it would if they were still together. He's potentially being opened up to paying £1000 that he can't afford as a result of her actions. If she is so keen to go away then she has to bear all the costs, not expect her ex to fork out! (not to mention a potential cout appearance and fine, and any ramifications of those).
How many people would be happy to risk a £1000+ fine as a result of their ex doing something that they didn't agree with and that also was illegal.
(apologies if this cross posts with anyone - started this ages ago and got distracted by the dc)