I don't know the backstory here but from what this thread states it seems that:
You had a row with DD3 and as such sent her to her dad's because you can't cope. Your elder dd went with her to look after her.
Subsequent to this row your ex has now stated that he's not going to be returning the DD's after the weekend.
And now you've decided that you need to see them and so are going to go up to the school because you have no idea where they're living, even though this was apparently not so important at the weekend when you were shipping DD3 out to her dad's because you couldn't cope.
Oh, and you don't think the school should tell him that you're coming down because "he's not even on the birth certificate," even though, again, this apparently didn't matter when you needed him to take DD off your hands.
It seems your children lack any kind of consistency. You can't expect to send them to their dad's one minute and then get annoyed when he doesn't think that being with you is in their best interests.
If this was the other way around and a man was doing this people would be advising the woman to get legal advice and to keep the DC away from this toxic man.
You need to take a step back and stop using your children as pawns. Don't go down to the school, take a break from all of this, and if your DS is six then he doesn't get to decide he fancies being an only child....