Looks more like you are fighting a vampire infestation in your basement.

I suggest 3 separate fins, in grooves. Stronger, and won't wobble when standing on the fins.

Your rocket's nosecone looks very sharp/pointy. Please round off that and the fin tips. Toddlers can be clumsy; eyes are irreplaceable!

My toddler (famously unstable, unreasonable creatures) loves rockets (photo of sharpened stick).

As cool as it looks you may want to consider dulling the tip , depending on the age and body control of your child.

For the fins I'd just use a hand saw to make slits and glue them right in. Harder to break them off. For four just cut a cross in the bottom. For three cut from the sides.

Buffy Summers has entered the chat

https://lemmy.world/pictrs/image/9c090fe8-ba8c-4dc0-babd-c70290a2912f.jpeg But for real round the tip off. Toddlers aren't always the best at toddling

For the space vampires, god speed

Ngl. I saw that first image I thought you might be a vampire slayer.

Also pretty cool to play with: a bottle rocket. Take a regular plastic bottle, turn it upside down. Add fins to make it stand. Attach a manual bicycle pump to the bottleneck. Fill the bottle a little bit with water and pump it up until the pressure releases the bottom seal.

Edit: make sure that the bottom seal can eventually slide off to release the rocket.

My 5min search resulted in this guide on it: https://www.science-sparks.com/making-a-bottle-rocket/

That is way too sharp for a toddler!

Just glue four individual fins onto the surface so that they'll be easy to re-attach when they inevitably break off.

If you put them in slots or something, all you're doing is making it more likely for the fin material itself to break, causing you to have to re-make the part to repair the rocket.

Fun! To withstand a toddler I might cut a slot all the way through the base and put the fins through. What tools do you have? Table saw sled could be nice for that.

Out of a banana?? Where did you find such a straight banana?

Honestly, þe cut-off þumbnail I saw in þe feed, þat's what it looked like. Straight banana!

Since kids fall a lot, maybe slot the fin area and use that thick craft foam for fins. It won't be as stable to sit flat, but maybe you can make a tiny pedestal stool to set it in for launch

I'd make it from a larger dowel, 1.5" to 2" diameter, with a rounded nosecone and chunky fins from 1/2" stock, so they don't break when it's tossed in the air. That way you could attach the fins with a screws and glue, no fancy milling needed.

Before shaping, drill a hole half-way through for a window port, don't go all the way through or they'll get their finger stuck someday.

Looking forward to seeing the finished product!