There are lots of childrens instruments available and I have been through a fair few of them myself. I usually find them to be out of tune. Trying to teach a child an appreciation of the finer art of playing an instrument when the instrument doesn’t sound very good.
We were sent this Plan Toys Melody Xylophone and although very simple the notes on this xylophone are true to scale and sound fantastic. The music sheets that come with it are a good place to start although some of the songs are not very well known.
If you didn’t know how to read music you may find it hard to explain to the children the difference between a solid note and an outline note on the music sheets as these are not explained anywhere and if you get additional music sheets for it that would be fantastic.