I think £100 a day sounds a lot. You can manage with much less than that, if you need to.
As long as your not eating out in the seafront restaurants every night (lovely as they are!), you can get by on much less than this.
We came with our 3 daughters a little while back - ate breakfast in the apartment; a light lunch; and a decent meal in the evenings (but in the smaller, away-from-the-seafront restaurants). Few beers every day (plus plenty in the fridge!).
Its the trips that cost the money and soon add up, but if you're on a tightish budget, you only need to do a couple each week. (Most of the boat trips are about a tenner each). The weather will be great, and kids will spend ages playing in the pool, which costs nothing.
As Alan says, bring a few hundred pounds with you, and cash it as you need it. Plenty of cashpoints around. Have a great holiday.
Julie
Oh yeah, and eat in the Calis Beach Restaurant on the seafront one evening and get free waterpark tickets which the kids will love! (I should be on commission from them - I'm always recommending both the restaurant and the water park, lol).