Hi cee cha,
There are some wonderful beaches all over the island and if you look hard enough you can find an area that seems undiscovered by the madding crowds that gather each summer. Figurettes as mentioned has an excellent beach, and the water is shallow at the northern end of the beach (before the rock jetty), going out to around forty feet or so from the edge of the shore before it gets to deep for small children to stand.
If you get the chance hop onto one of the island ferries and go over to Formentera and check out Platja Illettes, its very easy to find just head up from the marina where your boat will most likely dock and keep crossing the marina till you see a small path at the back of the scuba shop. Follow that and it will lead you to Platja Illettes which is a clothing optional beach and is very popular with young familes, especailly the Spanish from the islands.
The water here is very warm, much warmer than on Ibiza most of the time and is a very bright azure or turquiouse colour that is amazing, but best of all it is so shallow that even toddlers can paddle in safety under the watchful eye of mum and dad of course.
If you need any more information please just pm me or send me an email.
Hope this helps,
Marc