On our first trip to Disney (2018) my younger son was 3. I hooked his magic band around my purse strap, and just told the CM's I was tapping in for him. You can adjust the band size, but I was worried he would lose it or pull it off, since he's not used to having something on his wrist. My 5-year-old kept his on without an issue.
We went last week and our 3.5 yr old refused to wear it. I stuck it on my tote bag strap. When walking in the park the CM would ask how old she was. I said 3 and just scanned the band and my finger for her. No problem.
I traveled with a 2 year old this summer and he knew to grab his magic band when everyone was getting ready to head out to the parks. Since he was only 2 his band didn't do anything but he still wore it. I'd say be ready to go either way depending on the reaction of the child.
I still carry my 7 year olds...I definitely wouldn't let my 3 year old wear there's. It's just a pain to have to replace it and there's literally no reason for them to need it out etc., other than to get in the parks.
My middle son wore it and never had any issues keeping it on. Only my oldest has lost his in the parks, and thankfully I brought a back up with me. My youngest is 3 now and I will let him wear his but will bring another backup just in case.
We have quite a stash from several resort stays over the years, so I let them wear it as soon as they need it for admission. I bring a few extras with us on our trip for each person & I always keep at least one extra band for each person in our family in our park bag just in case someone loses theirs while we are in the park. In the past we’ve had good success with the Fitbit sliders to keep the bands from falling off. I caught a sale on shopDisney earlier this year for the slap bracelet style of magic band and some of the puck keychains, so we’re trying them out on our next trip.