Family Holiday Spain,Italy,France

Sister wants to go away end of June with hubby and 2 kids, 6 and 4, she would like to go some where with flexible arrival dates to coincide with Ryanair flights, entertainment for the kids, pools kids club ect.

If you want yr kids to meet other kids , and be entertained , kids club etc . Suggest you forget about Ryanair and get a package holiday . You could book a private accomodation on a nice complex but the UK schools close later than the Irish schools so at end June you may not encounter many families with similar aged kids.
For somewhere slightly "different" with short flight and familiarity of Spain try "Pueblo Indalo" in Mojacar- Ryanair fly into Almeria or perhaps Salou area which is close to Port Adventura.