Hi and welcome. Make sure at first you only get the essentials. Figure what extras you need as you go along. You mention kids, what ages are they and where will they sleep. If old enough, as my kids did, they preferred a bedroom annex in the awning. If you are going for an awning, dont rush. Have a good look around, there is a huge range of them and a vast difference in quality. You will certainly need a water barrel and waste container. Chocks and ramps for levelling, and a small spirit level. Consider what electrical items you will take and the total amps they will use, most caravan parks have either a 10 or 16 amp supply, but some can be as low as 5 amps. Too much drain can trip your pitch supply. As has been said many times too, the potential for bacteria in your water system is high. Many of us take a seperate container to fill with fresh water from the site supply and use that to drink and fill the kettle with, using the van supply for washing with. Are you planning to use a gas bbq in the future. If so, check that you have an external BBQ point on your van. If not, your dealer can fit you one in no time. Again, if you are going to use an awning, check for an external 240v socket. Some budget vans come with neither.
Above all, dont forget to insure your van and use a good company. The caravan club is a bit more expensive but you get what you pay for. Join the club and take a look at the clubs breakdown service which goes way beyond normal AA/RAC levels. An alternative is Camping and Caravan Club. Make sure your car insurance covers you, though it would be unusual nowadays for that not to be the case.
One big thing to throw in here for somebody new is learn how to load your van properly which really is a whole new topic.
The rest of the list of what to take is a large one and everyone has their own thoughts and ideas. If you get the main stuff sorted out, there are caravan accessory shops everywhere. You can always pick stuff up as you go.