According to my research there are about 10 different types of surface for tennis courts, my kids want me to buld them a court at our dzalka, so it would be good to get some feedback.
Grass is going to be by far the cheapest and easiest, plus it can be used for other things (croquet, for example, which is an infinitely superior game).
I reckon your best bet is going to be telling the kids that you'll start off by putting in a grass court for them and then see how much they actually use the thing: if they use it lots, you'll give serious consideration to putting in a hard court for them. Given the summer weather here and the fact that the court will only have fairly light use, you can most probably get away with grass permanently (my school used to have a few that lasted just fine even with lots of use and English summer weather).