My apologies for the delay.
At 10 the chid is too young to decide wether or not she goes for contact - and by offering alternatives the mother is in fact in breach of the order and your son could take enforcement action against her if he wished.
In addition your son has Parental Responsibility for the children and it is HIS choice where they stay so long as he does not endanger them in any way.
Your son needs to be very calm and not let his ex wind him up.
A picture of the chidlren with him in mediation may help him focus on what is important
He should be clear that in future they WILL be staying with his new partner, and that it is non negotiable.
If he can collect them form school all the better - and he could ask for an overnight midweek if that would work better
What he is asking for is reasonable and he should not allow her to make him feel he is not.
At 10 we make children do many things - got to the dentist, school etc - so she cannot say it is not possible to ensure the eldest attends ocntact.
Your son may find this website helpful
I hope that this is of assistance - please ask if you need further details