I error on the side of caution and always get a model release. It's contained in my contracts but I also get one if I use photographs that will be available for public viewing. If you're selling photographs, suggest always get a signed release. Of course, my primary photography focus is with people. I post very few photographs on my web site as I consider them private; they are for my clients. That's me; it's my philosophy, my way of doing business.
Why not get a model release? Is it hard to obtain? You may never need it, however, if you are, heaven forbid, taken to court and you don't have a release, then what?
Children are tricky. Suggest getting a release signed by both parents.
Years ago I never thought to take these measures but today, it's a different story.
Hope this helps you.