We didn't camp or caravan when our children were very young, but once we started, when the youngest was about five, we wished that we had done it much sooner. So, I just want to say, welcome and I do hope you have many years of enjoyment with your family.
When our daughters were younger we had a double bed at the front of the van and four bunks at the end, which worked fine. Now, we sometimes take our two granchildren (18 months and 3, and ususally put them in the end double until our bed-time, when one is moved to the front double with granddad, while the other cuddles up to granny down at the other end. When we lend the van to our daughter and her husband, they find the end double is actually big enough to take all four of them. So - just go for whatever feels best to you.
As for the gas fire - we have always avoided using it when we have had the children around, preferring a small electric fan heater. But, of course, that only works if you have a hook-up!
It's good to know that families are taking up caravanning. There is so much anti-child talk on this and others forums, but I can honestly say that in twenty years, we have never had a problem with children on a campsite. When we go away now in low-season, there really is something missing if there are no children around.