We drove from South Wales to Lake Garda last Easter, had an excellent time, and the drive was all part of the holiday. I have 2 dd's, aged 7 and 10 at the time.
We went on the shuttle - Folkestone - Calais, and had our first overnight stop in Reims (partly because it was convenient, partly because I spent 2 years there as a student and fancied going back). We stayed here, (the review in English is me), and it was great.
On the second night we stayed in Lucerne here, which despite the so-so reviews on the site, we found to be fine for what we needed.
When we got to Lake Garda, we stayed at this campsite through Siblu (we used Tesco clubcard vouchers) - would definately recommend it, and also would recommend booking direct, I haven't checked lately, but when I was looking last year, I think it worked out cheaper. It is a massive site, with everything you will need, but not many British people (you may think this is bad or good!). It is on the shores of the lake, and 15 minutes walk from the nearest town.
We managed a day trip to Venice - about 2 hours on the train (I think), also went to Gardaland for the day, as well as up monte baldo in the cable car.
On the way home, we stayed in a hotel near Mulhouse, and then in Reims again.
As you can tell, we had a fab time, and would heartily recommend it to anyone, as long as your children are good travellers...