Hi Mark, for a family friendly beach I think the Mar Menor is fantastic and I would recommend staying near Los Alcazares.
Perhaps the best urbanisation in the area to stay would be Roda Golf. There are plenty of apartments to rent at very good prices and the resort is really well maintained. You will also be able to use the communal pool, however please be careful as to which pool you are allowed to use.
As you daughter is 3 years old the sea at the Mar Menor is wonderful (and just 5 mins drive) and shallow and some of the communal pools have areas suitable for paddling / play. There is nromally a funfair opposite the urbanisation but it can be expensive.
Please also make sure that the apartment has air con as you will need it in the summer.
Downside is that it may be near Murcia?