I have two children a girl aged 13 and a boy aged 11.
i work six days on a then have three off, my partner does five on three off.
My children spend the time when I am working with tHeir father (who doesn't work) and then come to me on my days off.
We then have them for three or four days, more when Im off and during school holidays.
This is a mutual agreement, but eventually later in the year we want the children full time as they will be at the same school. And they want to be with us.
But the fathers is getting tetchy, and says we/I can't have them this week, because he's got wind of the full time with us and is worried because he will lose the housing benefit, child tax credit extra he gets.
Are the courts involved, or is this an informal agreement? I don't know, but I would think that you can't just move the children to live with you without his agreement, if you both have right of access and custody.
If the children want to live with you then they are both of an age where they can choose for themselves
Yes, your ex will lose his benefits (but I dont see that as a bad thing at all)
He cannot stop you seeing them and vice versa
If it were me as it has been mutual up till now I would try to talk with him, if you cannot agree then you could try mediation or a solicitor
Start with the Citizens Advice Bureau. A lot depends on whether there is a court order, and what it says. You may have to talk to the Courts' welfare officer, who may wish to speak to the children.
You will have to involve solicitors sooner or later