So it's happening!
Well, I've just had a wee look at the website and I think St Jean looks like a really nice place to live.
Healthy mountain air, things happening, stuff for kids (trampoline? I'm ), lovely part of the country, access to TGV for trips around France/back to the UK...
When you're packing up, keep aside some of your & DC's most important things, rather than sticking them in your removal, so that you have them available right from the start. Oh, and take the time to label your boxes well, to save annoying hunting around for things.
It's hard when you have young kids, and I haven't managed it yet, but joining a club/group of some kind makes a world of difference to your social life.
If you're still not that enthusiastic about it all, try and sound a bit excited about it and stress the positive sides for your kids, so they don't get too stressed by the move. Faking cheerfulness makes me a bit more cheerful than I would otherwise be.
IIRC, you have family nearby; if you get on with them, exploit them shamelessly, to help you meet people, look after the DC for an hour so you can get time to explore your new home, etc. I really regret not having that opportunity.
Don't know what Belgium is like, but be prepared for amazing bureaucracy, requiring endless papers and detailed knowledge of your mother's maiden name, your last 3 payslips etc.
Good luck...