Hi @Peachesnocream. Please don’t take some of the responses on here to heart. You’re clearly a very good mother in a difficult situation. And you don’t want to make things worse and lose majority custody.
He’s clearly not being a good father by any reasonable person’s reckoning. He should be giving them bedding, comfort, enough food to eat and his full attention. Or he shouldn’t have them on a regular basis. He can clearly afford it if he keeps going on holiday. He shouldn’t be leaving them with other people on his one night with them, especially his gf family. What if her mother is cold and nasty, has a dangerous dog or a man in her home who’s abusive?
Remember, abusive people can manipulate children by telling them something bad will happen if they tell their mum.
I strongly suggest you seek legal advice. It’s very concerning he won’t give you his address. Unless there’s a history of you turning up at his address etc, that’s not positive coparenting. It shows he’s being secretive. So, what else is he hiding? Could he be living in a homeless hostel where he gets a self contained room? Does his gf stay over when the kids are there, i.e. do they have to hear them being intimate? If I was seeing a man who had his kids one night a week, there’s no way I’d expect to see him on that one night. I’d give them space and time together. So, the gf also sounds quite thoughtless to the kids. Is it possible he’s even gone out (when no one will look after them) and left them on their own, but sworn your daughter to secrecy?
Given the children aren’t being put first, your DD may be very stressed about her younger brother. If he’s 4, could he be crying from cold or hunger and she has to mother him? Could it be he’s not being looked after in other ways, so she’s worried he’ll hurt himself?
Is your ex the sort who’d bother with a court order? It’s costly. Are you sure he’s not just using 50/50 as a threat? This sort of scenario must be quite common. I strongly suggest legal advice, even if you have to borrow the money to see a solicitor.
And don’t believe your MIL saying you’re being dramatic. You’re absolutely not. What mother would want her young children staying at an unknown address, being left with strangers (again, presumably, at an unknown address), having to sleep without bedding and not even being able to eat toast without going out. A toaster from Argos costs about £10 FFS. It would be bad enough even if your DC are happy staying there. But your DD clearly isn’t.
I hope you can get advice. In the meantime, could you buy them their own ready beds to take (airbeds with built in sleeping bag)? And pack them healthy snacks (dried fruit, pastries for breakfast etc). I know it’s his responsibility, but if it means they’re more comfortable while you go for full custody, it’s surely worth it. Ready beds may not be a total waste as they could be handy for friends sleepovers at yours in a few years.