Is it not worth having a rethink?
TBH I can't see how a 1 yr old baby is getting much out of Centre Parks in December or even the older child. Mine enjoyed it most when they were around 8-12.
Those days could be used for a week in the summer holidays.
Likewise your trip to Wales- is this to see parents or in-laws? Could they not come to see you for a change?
If your H gets 25 days holiday, taking 10 over the summer so he is around for 2 weeks still leaves another 15 days.
Why don't you try to carve out more time for yourself?
It comes over as a bit of martyr syndrome going on.
I'm sure your DH could cope with both children for a few hours at a weekend.
Then, you could go shopping, have a spa trip, do whatever you enjoy.
Your kids don't need their mum 24 hours a day when they also have a Dad around.
Are you afraid to leave them with him for some reason?
If your MH is suffering by being with them all the time it's within your control to change that.
Maybe this is a conversation you should have with your husband.