Anything I can do?

I've just split up with my long term partner, we're going to split time with the kids between us. When we were together he did little to no childcare even when he was working two days a week so I don't know if he would but I have this nagging thought that he could take the kids for the night and then just not give them back. Is there anything I can do to prevent this happening? I'm probably just overthinking but I have heard lots of stories of things like this happening. I'm in the UK.