well, you need to decide if you want to stay the night in a hotel and break the journey. pros - 2 shorter drives, less stressful/boring, etc.
cons, cost, dragging journey out and in effect loose half a day holiday, possibly uncomfy night in shitty motelly type thing
I personally rather get the journey over with, but then mine are all v. good travelers, thank goodness, but probably because we've had to do lots of long journeys.
If you decide to get it over with, then an overnight drive would be best, which counting backwards, you'd be looking at leaving Calais about midnight, which would mean a tunnel crossing at about 10ish at night. Luckily it's winter so it will be drak, not so good in summer when sun's up at 4 or 5, so dc should remain sleeping all way there.
If you decide to break journey with overnight stay, I'd choose somewhere about 5 hours from Calais, so you have 2 x 2.5 hour drives, and aim to arrive at that place around 5 or 6 so you have plenty of time to unwind, have a play, have an evening meal in restaurant etc before bed at normalish time. YOu'll have to work out timings backwards form there, but these are the 2 senarios, up to you to work out pros and cons.
Still v. young, but if you have the forward seat by then, you could consider getting in car dvd player, ours is a godsend, but then my kids are older, but it really really makes a huge difference.
hth