I guess it depends whereabouts you're after - but we went here a couple of years ago (DH, me and 2 young DC). 3 beds, nice swimming pool (not enclosed), nice outside bit, enclosed, gated garden, children's bit (older children really though), nice lady owner lived onsite, but out of sight iyswim. And a hot tub!!!
Its in Oliva which is half way between valencia and alicante (45-1h either way). Its a more Spanish resort, although not very touristy and pretty quiet.
You could also look in javea (or Calpe) which is very nice and bit more going on.
Sorry...just realised you actually asked for website recommendations!? You could try www.hpbtenancies.co.uk HPB is very reputable, not the cheapest, but you're generally safe in that the standard of accomodation, kitchen well equipped, well looked after properties and the area are going to be good. its not just spain, but does have some spanish ones - we've been to one of the javea ones - which was good.