Advice regarding contact orders

[user1482413972]

My ex & I had a huge row over Christmas period basically I made arrangements to visit my family with our kids and the day we return is meant to be on one of the days he has them, I didn't ask him I just arranged it as I assumed (rightly or wrongly) that as it was Christmas he would be ok with it, well unfortunately he had made arrangements to take them away for weekend and was was extremely angry about not being informed.
Anyway, he has since sent me an email stating that we need to go into mediation first week of Jan as first step towards him obtaining a contact order as he's 'fed up with me making arrangements when he is supposed to be with children & my general flaky demeanour when it comes to shared contact'!! TBH I very rarely make arrangements on his time however it has happened occasionally but usually it's due to clubs or parties for the children. I'm not sure what to do now or where to get advice or should I just refuse & ignore it. I actually believe this is more to do with his wife than him only because she can legally advise him but regardless I'm still shocked and have no idea how this process works. Another irritation is that I will have to pay for the mediation even though I didn't instigate it which I find ridiculous!
Advice please, thanks

When did you let him know that you were taking the kids away in his time? You have admitted that more than once you have prevented him from having the children at the previous agreed times. I am not surprised he wants to make things more formal.

I text him about around christmas eve, maybe a day or so before. I haven't prevented him I just decided that if the kids want to do something else then they can.

Well I think you have been very unfair on him and the DC! His time with the DC is his time and if you would like to alter it for any reason you ASK and only do so with his agreement.

Mediation can be a cheap simple means to formalise the agreement you currently have to avoid having to go to court. It sounds as though you just need to respect the agreed contact times.

How would you feel if he just decided to return them late at short notice and it meant you had to cancel a planned trip away to visit friends/family?

How old are your children? You can't make the decision that the children can do something else on his contact time. If the kids want to do something on his time then they need to ask him and make sure it fits in with his plans.

Turn it the other way - would you have been happy if he text you on Christmas Eve letting you "know" (not asking) that he was picking the kids up a day early because he'd made plans to visit his family with them?

If they have parties etc then just let their Dad take them instead of you.
If you have agreed days /times then you should stick to then. It's not always ideal for xmas or holidays etc but you need to suck it up and stick to arrangements