My daughter grabbed the inside rim yesterday and cut her finger, even though I thought I checked to see if it was smooth enough - guess not - so I would suggest sanding the inner rim down before giving it to your kiddos.
The other thing I did differently is I used letter die-cuts I've had for several years instead of printing them on the paper with the computer/printer.
The above picture didn't show it, but later I cut a slit in the lid so he can drop the letters in. I just used an exacto knife to cut through the plastic lid - worked great.
I showed it to my son when he got up from nap and there were a lot of wow's and smiles. He's just learning his letters so he loved seeing them on the metal lids that he could drop in the jar.
Then a few weeks later, I saw on her blog that she made something else with juice lids . . . a color quiet book. So now, I guess I have to go out and start buying juice concentrate to get the larger lids.