Just let's take this back a notch. You're getting tied up in the arrangements for the day (understandably).
But....I think you need to take a massive step back and consider :
"What is in the best interests of the girls?"
"What about this day, will be a positive experience for them?"
I would say, that they would be far better off, giving this wedding a miss.
They are too young to appreciate what a wedding really is. They will have to listen to stupid speeches that gloss over how inappropriately this wedding came to pass. They will have to watch Daddy get cosy with a stranger.
It's just too confusing.
What's more, if you were to do something else really fun on that day, they wouldn't even think about what's happening some 50 miles away.
I honestly wouldn't let them go.
If their Dad trots out some bullshit like "It's my wedding day, it's so important that they are there", then you reply with :
"Weddings aren't important, it's just a piece of paper" 
If finances allow, I'd even book a holiday abroad and grab a lovely week of sun with them.
Regards the school fees, be careful here...what will he have to pay instead, by way of child maintenance? If the school fees are more, then let him stick with that. DO NOT let him away with paying nothing. Also, if you had a joint account up until the split, and his wages were paid in to that, ensure you keep bank statements, as proof to CMS of what he was earning, just a short time ago. You may think that he'd never try to under pay. I thought the same, and was proven very wrong, coincidentally, as soon as he got with a new woman, who didn't like him paying. He got away with convincing CMS that he earned around £32,000, when infact, he was earning £132,000.
It's obvious he can be a total bastard, so never underestimate what he may be capable of.