Hi Pixie
This reminds me very much of my own experience with my ex. Despite me being main carer and her having the kids every other weekend, one day she decided that she wanted to be full time mum again and failed to return them.
Your priority right now is to get your kids back under your care. Do not leave them with your ex. Turn up on Wednesday. If need be, take a friend with you. Preferably a big male.
The concept of fairness should be thrown out of the window for now. Do whatever it takes. Pick your 4 year old up from school and given the time, keep her off school.
Your solicitor if he/she has any brain cells will then write to him saying that contact is suspended pending an emergency court hearing. But "emergency" hearings may still take weeks. Mine took 3 weeks for an initial hearing. During which I watched the kids like a hawk.
The reason for this is because ironically the longer he has them for, the better his case.
At the court hearing, chances are a temporary court order will be issued. Ask your ex to sign an undertaking that he will return the kids as per the court order. This is voluntary and courts cannot force him to do this. Basically if he fails to do so, he is instantly held in contempt of court and it is arrestable (this will be made clear to him). If he refuses to sign then well....that says a lot to the judge doesn't it?
The way you need to approach it right now is that this is a war. He started it, you will finish it. Shock & awe is the order of the day. Because frankly he won't think twice of screwing you over if he can. My exGF told me that when I was fighting with the kids mum. I still tried to be fair & reasonable. Trust me, it doesn't work. My ex was right on that one.
So in short, no more contact with the ex until tomorrow. Turn up to his house tomorrow and assuming he's in, get your backside in there and get the kids back. Information is power.
Trust me, usually I'm very pro-dad and pro-contact. But there comes a time when talking is over and action is needed.
Good luck