I would advise againist it.
Little kids get cold to easy.
A ducky is very wet cold ride compared to a raft.
Maybe on a very hot august day with low flows.
I think it would be significantly safer and more fun for the kid to do it in a raft.
Note....not a single person has advised you that it is a good idea.....doable ....yes it can be done in reasonably safe manner, right day, right water level, right people backing you up.
When the kid gets a little older (8-10) and is a good swimmer, you can get him his own ducky to paddle !
Note.....I took my 9-10 year down Browns in tandem ducky in august on a hot low water day, with rafts in support.....we did fine and had a good time.....but he was a good swimmer, had run browns before in a raft, and had driven his own duck, for parts of our gates of lorde trip. In fact when he was 12 he solo ducky all of our gates of lorde trip that year.
He is now a senior in high school, and r2s class IV water with me, even calls paddle commands now.
I started him slow with motorcycles as well....litlle bikes, age and ability appropriate trails and rides.....now he has his motorcycle license, rides a platted KTM 450exc with me...in 2 weeks we leave for spring break ...moab bound with our bikes....later this summer we have a 10 day dual sport adventure camping ride planned.
Point being start slow, put him in swim lessons or swim team....and you to could end up having the best possible R2 partner
Enjoy your little one