We have been to a place called Morrungulo. It is an awesome campsite, and they also have chalets (at a price obviously). It is a full day's drive north of Maputo, and as long as you have a vehicle with a bit of ground clearance, you should get there no problem. There is about a 15km dirt road from the tar to the resort.
However, you do need to be FULLY self contained. The only thing you can get there is bread, maybe some fruit, and obviously fish and prawns. Electricity runs off a generator for about 10 hours a day, so you will need either a gas fridge, or one that can run off a battery.
The beauty of the place is that you camp almost on the beach. Security is top notch. The ablutions are good, and the place is fully grassed (not normal for Mozambique), so the family will think it's great. There are no major dunes to go up / down to get to the beach.
Stumpnose aplenty(2.5kg upwards), and kingfish too. And that's in July, which is traditionally not the good time to be fishing. Also ideal for fishing from a fishing ski. It's also about 90 min's drive from Pomene, which is a killer fishing spot (but you will need a 4x4 to get to Pomene)
Check out
www.morrungulo.co.za
Ian F