hi, just like you we live in Lancashire, have two kids (now 15+11)and we have been to holland 3 times now. It really is so easy and for your first time it is ideal. We have been each time via Hull to Rotterdam. It is an easy route, good motorways and well signposted( there's a couple of big hills, but nothing to get worried about. you can save money by going to Harwich, but that route increases your travelling time both in this country and in Holland. Going via hull, approx 4hrs drive from Lanc's with a food stop on the way. You can get on the ship from 1730 onwards usually, you have a cabin and there is plenty to do. Younger kids have a play area, there are bars and restaurants. This really chills you nicely for the overnight ferry.
Caravans are put in with the lorries, and they don't like hanging around when you dock in rotterdam. We recommend the breakfast deal, sets you all up for the day,buffet style, buy the tickets when you board.
We use Koningshof campsite, in the middle of the bulbfields, lovely!!!! This site is a bit further than Delft but i think is owned by the same people. It takes us no more than 2 hours to reach the site. They are nice flat roads, good motorways, you have to be carefull checking the road signs (we've had to do U-turns in some strange places). We love it, and some of our best holidays have been at this site. Cycling in this area is fantastic, there are some beaches within 4km. The pools on this site are brilliant for kids.
Both the clubs use this site, and can arrange a package for you. I can't praise this holiday enough, and will certainly be going back in the future. Any more questions please ask.
stevec