Eurotunnel - 35 minutes. You can get out of the car but there isn't anywhere to go - there are about 4 cars in each section, iirc. But you can sort stuff in the boot or wander around the car - it's quick. Haven't done it with dc.
Ferry - we did Poole/Cherbourg last week - it was about 4 hours. It was ok. For us, it was much closer to our destination - a 3.5 hour drive as opposed to a 7/8 hour drive from Calais. Were overnight on way out and shared the boat with 6 school trips on way back!!!