Tricky situation really, you have a young child who is being unduly influenced by the other child and by mum.
There is the potential for the situation to become more inflamed you realise I'm sure.
Could you re-iterate what the Court order is for, what the parenting schedule is (and for which children).
When was the Court order made and was it at magistrates/county court.
Did the ex have legal counsel (i.e. solicitor)
Any Court orders made after Dec 8 2008 automatically have a "warning notice" written into them at the bottom warning the parties about the consequences of breaching a court order and also the powers available to the Court to remedy a breach (i.e. ordering parenting activities/parenting courses to be attended, community service in the most extreme cases).
If your Court order was made after dec 8 then it is a simple matter to apply to Court to get it enforced - you complete a form C78 (available from the hmcourts website), complete it, file 4 copies at Court with the appropriate fee (£40). A warning is sent out to the other party about the consequences of breaching the order.
Hopefully they take notice - or they dig their heels in more.
Then you complete and file a C79 form (application for enforcement of an order) with again, the appropriate fee, the Court should indicate what course of action it proposes to take to enforce the order. This will usually be by listing a hearing - i.e. you have to attend Court.
... if your Court order is prior to dec 8 2008 then things are a bit more tricky as you have to either write to Court pointing out that the ex is consistently breaching the Court order and request that the Court attach a Penal Notice to the order (i.e. the equivalent of the warning notice that is automatically on all 'new' orders). Many courts for some reason are loath to do this and you may find them either refusing - in which case you will probably have to apply to Court for enforcement of the order, or the Court may list a hearing for both parties to attend.
... so no matter what you do it may end up at Court again. This is no small thing to do (having been through the family court system myself self-representing and no desire to do it again).
Is it possible to try and engage the ex somewhat? Maybe approaching your local family mediation service initially to get them to invite the ex to Mediation - this will at least show willing rather firing off to Court first.
Also, get yourself along to as many local family law solicitors (preferably those that are members of "Resolution") and get a free initial 1/2 hour consultation to gauge their opinion.
I wonder if the ex will hide behind the "It's not me, it's what the children want" mantra (forgetting that if you did everything a child would want they would eat chocolate all day and not go to school)
I would also recommend you get along to your local Families Need Fathers branch to get some practical face-to-face advice