It sounds like you're dealing with a difficult party, which means unfortunately, if they want to turn it into a back and forth debate, they will.
So what are your options?
The final goal should be to get the final orders varied so there's no room for misinterpretation - have a new order included that clearly states what happens after the school holidays conclude.
In my view, there's two possible ways to go about this. The first is the usual path of having a solicitor emailing a suggested interpretation, then organising mediation if she's non-receptive, and then filing an initiating application to have existing orders varied by including one ancillary order for clarity. I think the Court will support your position over the mother's - it's an ordinary meaning type deal because it's a fairly standard order for the Court to make.
I also think the Court will nip the issue in the bud quickly - it won't want you in Court over such a minor dispute for any longer than is critically necessary, but also, who knows? Maybe the prospect of more proceedings will compel the mother to change her view.
The other (potentially more hostile) way is that you could try and have it done through contravention orders. Send a letter clarifying the ordinary interpretation, notify that any deviation from this interpretation will be treated as a contravention of the orders, request assurance that she will continue complying with the orders and state that the father will collect the child at 5pm on [date], in accordance with the orders.
Attend on the mother's address to collect the child, if she refuses to facilitate the child's time with the father, then calmly leave, document what happened, then wait two weeks and do the same again, and then again two weeks after that, and then file for contravention orders on grounds the mother has failed to facilitate the child spending time with the father in accordance with the orders.
Even if the mother can establish a reasonable excuse that removes her from wrongdoing, the Court can still exercise its power to amend orders as part of contravention proceedings, and again, I think the Court will be unlikely to allow the parties to leave without clarity that might reduce the likelihood of them ending up in Court again.
I do suggest getting legal advice on this before acting, but those are the paths I, personally, would consider were it me in that position.