Just did Deso in October at low water and have also done it at medium and high. I'd say it's a good, challenging open-canoe run for practiced adults (with full flotation). Over the years, my canoe-loving mates who've done it have dumped at least 2-3 times, most often in Steer Ridge/Surprise (a sneaky hole), Three Fords, or Coal Creek Rapids.
4-6 year olds are too young for open canoe trips on Deso at higher water. You might be okay at low water, but the chances on bonking a rock are greater. I'd wait 'til they're a few years older.
Labyrinth/Stillwater or Ruby/Horsethief would be good choices. I have friends who've taken their daughter (7) down the San Juan in an open canoe. They dumped a couple times, the kid screeched and refused to get into the boat, and they had a big fight over whether it was too risky for a child that young. When your cub is involved, at the slightest hint of danger, emotions can run high.