We always self-cater and the past few years we've gone to France and driven our own car, because then we can take what we want. As your DC are young this would be a good option for you as long as you're confident driving on the continent (it's much easier than you think and French/Belgian motorways are much less crowded than their British equivalents). You could also go to Spain, Portugal, Italy, Greece, etc and hire a car (but I'd advise you to take your own car seats).
We always look for a place with: a separate bedroom for the DC, a DC-safe pool with a fence, a bathroom with a bath, a playground or garden with play equipment, sandpit, etc, and ideally some toys and books in the cottage/apartment.
There are masses of places that meet these criteria and several websites out there that are helpful: Baby Friendly Boltholes, Alistair Sawday's, Tots Too, Away with the Kids, etc. The Mumsnet travel reviews are also helpful for getting ideas, as are threads on the 'Self Catering Europe' board, where posters often mention places they've stayed, but haven't necessarily written a review.
My husband, myself and two small children 1 & 3 are going on holiday abroad or the first time. We normal go to Spain or Portugal and stay with family but this year we want to do it alone. Need advice on what to look for in a family friendly place. What it should have and what we need to take. Any help would be great and also thing that have helped mum & dad have a relaxing holiday too.