I thought of something you could run by your FIL. He's obviously loving the idea of a green lawn the kiddies will enjoy. But maybe he'll be open to allowing the addition of some California wood sorrel amongst the pointy grass. If mowed it stays at about the same height as the grass, it's nice and green, it's soft to sit on and the kids can hunt among the three-leaf clovers to try to find a lucky one with 4. If it gets the chance it will make pretty yellow flowers and then nifty seed pods shaped like miniature okra, when they are ripe you can twist to make them pop seeds everywhere. It also handles drought well. And it can help crowd out non-native weeds. But it could possibly take over the whole lawn. Which will make him mad if he thinks of it as a weed rather than a chosen inclusion. So if you think he might say yes, ask him. If you know he'll say no, don't bring it up. It's legitimately possible for it to start growing in the lawn all by itself because it's so prolific, and you don't want to be blamed. Especially if you did surreptitiously scatter some seed...