Hi,
I'm not going to lie to you - the last 2 weeks in July and most of August does get very very hot in Gumbet. There may not even be the cool breeze from the bay at that time of year. As has been said before, keep hats on the kids, and when they are in the pool make them wear a T-shirt over their cossies - they will want to be in and out the water all day long, and so sunburn on their shoulders, necks and back could be a problem if you don't keep them covered up or constantly slavering suncream on them.
You should be able to pay for air conditioning in your room when you get there, and I'd say at that time of year its advisable to do so, just so you can escape the heat and get a good sleep. If there is no air-conditioning at your hotel, you'll still be able to hire fans for the rooms.
If you're planning a stroll round the resort, maybe go in the morning after breakfast, so you're not walking long distances later in the day when the sun is at its most fierce? Or wait until evening - all the shops, restaurants etc are open til about midnight.
Elaine