Ask them what they want to grow to eat, help them plant it, and help them take care of it. It's usually best to start with one or two plants, so they don't get overwhelmed. Is it too late to try that? That's how I got my son involved. If you want help with your garden, then their children should be willing to help. That's not the same as trying to get them genuinely interested in it though. I'd make it a chore if they aren't willing helpers. Relationships are give and take. They'd best learn that now. I'm sure you do a lot for them that you'd rather not do.