Go for it! With small kids I would make it very shallow and fill it with pond plants. Since you live on the edge of a marsh you will have frogs and toads and the occasional snake. Don't worry about the snakes as they usually will not bite unless the kids attempt to pick them up. Even so I have been bitten quite a few times since I was a kid and am no worse for the wear.

As a kid my dad loved the outdoors and we were thrilled to encounter all types of "critters". It really helped me develop compassion for wildlife and it was such great fun. I think it would be great fun for your kids. Make it a family project and they can learn about responsibility as you care for the pond together.

As for attracting too much wildlife- I don't think that will be a problem. You already have racoons and they usually will not be out except at dusk and dawn. Same with possums. They might visit the pond and they might wreak a little havoc with the pond plants. So plant things that are inexpensive to replace. Also if you can add a small pump and create a small waterfall you will attract birds which are great fun for the kids to watch.

You would need to be very careful about allowing access to the pond for the children. Of course if there is a marsh you should be aware of the dangers already. You might consider a pondless waterfall which would give you many of the pleasures of a pond with less hazard.

Pick up a half whiskey barrel liner at home improvement store. Will be less than a foot deep but plenty of room for frogs, dragonflies and plants. You can arrange flat stones to hide the edge or use outdoor silicone caulk to glue on fancy glass or stones from craft shop. Sink into ground. Instant small wildlife pond. Pot plants in pea gravel so when raccoon trashed them you won't have a muddy mess.