Cattolica, or anywhere in the Rimini area of Italy. Unfortunately the only airline that flies there at the moment is Ryanair, and only from Stansted. But if that's handy for you then I'd definitely recommend it. For a start the Italians LOVE children. They go out of their way to make the child (and you!) feel welcome. The beaches are mainly privately owned, so you do have to pay to go on them, but they have great facilities, with changing rooms, toilets, and kids play areas. At night everyone, and I mean everyone, parades up and down the main street. All the shops, bars, ice cream parlours, amusement arcades etc are open and it's a fantastic atmosphere.
Having said all that I never relished the thought of taking my little ones on a plane. We went for many years to Mumbles, Gower, south Wales. The beaches are fantastic, easy access parking, shops, toilets and they are pretty aswell. Plenty of playgrounds and other amusements in both Mumbles and Swansea (Blackpill Lido is great for little ones).