I did a similar project w/my niece and nephew [when they were 10 & 12 y.o. respectively]. Neither could get the hang of using a handsaw, so I introduced them to Mr. Bandsaw and Ms. Drillpress....& a hammer. I too, used poplar. Their project was to make (their own) bench w/back that was sized to fit them.
My niece had to stand on a box to reach the drillpress handle; but it became her favorite tool.
After the holes were drilled, I had them first, glue the bench together and then, screw it tight. [Yes, I used clamps.]
**I gave up on letting them hammer it together: too many dents, too many bent nails.
My niece opted for a clear finish. So, after sanding, a light coat of waterbased sanding sealer followed up w/polyurethane. My nephew discovered a small can of flat black enamel paint - I couldn't dissuade him.
While my sister, liked both finished benches, she told her kids [& me] that she had no room to spare in the house.
Really?!
So, I told the two youngn's that the next craft show I did, I would try and sell their benches.
And they would get ALL the money.
I still have the boy's. Folks were put off by the black paint. I was offered $35? for my niece's, barely an hour into start of festival [ greensboro 4th of july ]. I took it. The next time I saw them (they live out of state), I told my sister - Hands off! All the money was my niece's to do with as she pleased. Sis said, OK. ...(Really?!!)
As compensation to my nephew, I told him, he'd get $20 for xmas.
I showed my nephew [@ 14] how to deal with Mr. RAS... My niece is just too short for it; plus it intimidates her.
My niece has said she's built shelves and stuff for her dorm room. IKEA, I think. But she has that confidence to do it on her own,..and with only with a screwdriver. Imagine that.
My nephew has made some stands for his computer and gaming stuff. And they both helped my sister install wood flooring and helped with the staining/finishing.
So, to Rick. may you enjoy doing future projects with your granddaughter as she's growing up.