I have some experience with this. Three is possible with some children but not many (my opinion). Tune the uke WAY down but in tune with yours. This makes the strings super easy to fret! Sounds a bit buzzy of course, but who cares, their little soft fingers won’t suffer as much! Use different colored smiley face stickers and only one finger chords and half chords, I'll attach a short vid. Make the "C" a yellow face, "Am" purple, etc. But first focus on just holding the uke and strum down open. It's
very important to make a strap for them. Let them pick the material at the fabric store.
Play very slow songs and play with them. If they can't strum yet have them "pat" the ukulele to add percussion. Let them know how much they add to the music doing just that alone
Patience, patience you need tons of! Avoid the word "no". It's all yes, yes, yes, fun, fun, fun! Tell them Uke Time is rest time. This is a calm activity. It helps to put them to bed with it (I don’t mean in the bed but alongside or nearby). There's a chance when they wake up they will fiddle with it
You get them into bed usual routine etc. read, whatever, then you have them play you a night time song and then you play them one. This reinforces the “calm” when uking at other times in the day.
Oh, and I hope to gosh that you get them all a uke that is sturdy so you don't have to watch them constantly so that the uke is fully theirs (with rules of care, of course). I recommend the inexpensive all plastic KeAloha at Amazon. I have 2 of these. One plays very good the other is just so, so. But good for 40 bucks!