On a self catering holiday, the first thing you will need to decide is where you are going to go. The venue will have a large impact on what you will need to bring. Going to a water park will require much different gear than going to a forest location. The next thing to consider is your transportation there. If you will be flying, you will need to pack much lighter than you would if you were driving.
Second, where will you be staying? Often times, a self catering suite will have many of the conveniences you need when travelling with young children. If you inform them ahead of time, they will be able to make those conveniences available to you (such as high chairs or child's toilet seats). They might also be able to supply you with entertainment items, such as beach balls and the like.
When packing your vehicle for a driving trip, it's better to use a large rucksack as they are much more pliable and easy to stuff places than a standard suitcase. Plus, they generally have pockets for whatever you may have to pack at the last minute.
In order to have your packing fully figured out, you will need to decide what your daily activities will be. Try not to pack too many things into each day. If you plan your activities at an easy pace, you will be able to get rest and your children won't be so overtired that they become less than a pleasure to handle. As far as eating schedule and day to day activities, it is a good idea to stick as close to the normal as possible. Again, so you won't have children that are off schedule.
When planning your daily activities, check out travel guides, you can find many of them online. Also, when at your location, talking to the locals can reap big benefits as they can tell you about the places around town that are geared towards families with small children. You don't need to spend large amounts of money to entertain small children, sometimes a walk on the beach, or playing at the local playground will fill up hours of time for them and give you some time to relax.