Hi there, we did three Mark Warner holidays with our two before they started school. We loved every holiday. We went twice to Lakitira on Kos. Great kids clubs, food was good, lovely setting with 2 pools (one more family, one quieter) and you could do as much or as little as you liked without feeling any pressure. Kids went to clubs, I read by the pool and husband did sailing/windsurfing. Would highly recommend. Rooms were newly done and were nice. Afraid I can't comment on the other two companies.
Big hint from me - go on these kinds of holidays before your child starts school. After that, the prices are reaaallllly high!