Sounds like a difficult situation :-) Where in this area does it flood? At the top or at bottom of the slope? If at the top, is there something you can do disperse the flooding, like a dry creek bed that will direct water away when it accumulates? The reason I ask is that very few plants will tolerate periodic flooding so that may need to be something to address before you consider any sort of plants for the area.

You will also need to consider access to any utility boxes - they need to be accessible without you or utility personnel having to hack through a plant jungle to get to them. Ornamental grasses could work as could any number of shrubs, provided the drainage issue/flooding is addressed. And thorny ones are good kid deterrents - roses, barberries, flowering quince, hardy orange (Poncirus trifoliata), yucca.

Would putting up a fence help?

It floods at the bottom, but sometimes at the top too, depending on how full the bottom ditch gets. We're working on solutions to that as well, as that ditch covers 2 sides of our property. HOA won't allow a fence :( The other houses have the grasses, which works for their yards because their boxes are all close together, ours are more spread apart and there are 5 instead of 3. So the grasses would be nice, but would that be too much? The specific area is on a corner, so there is the side seen from the road and the side we see from our house. The access is all on the house side, so flowers would be easier for workers to deal with...maybe?

Flowers won't stop the kids - you want something large and relatively permanent. Even the grasses will need to be cut back annually, leaving the area vulnerable to trespass until the grasses grow back. But fix the flooding/drainage issues first.

Thanks