Stick with snorkeling fins for now. They're usually cheaper, smaller, and lighter. You kid will quickly outgrow the fins anyways.
You would probably be better getting rigid paddles. The Deep See Splash look like very short split fins. I don't think even several kicks would produce enough thrust for your kid to push his scuba unit and himself through the water at a decent pace.
The summer programs I work for uses Akona Caicos fins. They do the job perfectly. Very light so they don't hamper the kids legs but rigid enough to give them propulsion. We let kids snorkel on the surface with scuba backplates and Al 40's & Al 19's. If they have the skills to perform a decent fin kick they're usually cruising around very well.
I myself find they do a great job underwater, so long as you stick to flutter kicks. They're really too light for my liking though, but I think they work great for the kids.
