Okay buddy, so you know what a regular trombone looks like, right? It's a long, shiny metal tube that you blow into and slide up and down to make different sounds. Well, an alto trombone is like a smaller version of that. It's still a shiny metal tube that you blow into and slide up and down, but it's shorter and higher pitched than a regular trombone.
You can think of it like a big person and a little person both playing the same instrument. The big person can make lower sounds because they have bigger lungs and a bigger instrument to work with. But the little person can still play the same notes, they just sound a little bit higher. That's what it's like with an alto trombone. It can play all the same notes as a regular trombone, it just sounds a bit different because it's smaller.
And just like how you might have different sized shoes for different occasions, musicians sometimes use different sized trombones for different types of music. An alto trombone might be used in a smaller group or for certain types of classical music where a smaller, more delicate sound is needed.
So there you go, little buddy, an alto trombone is like a smaller version of a regular trombone that can still play all the same notes, just with a higher pitch!