My kid wants to convert his current ride to a SS because he's getting a new geared mtb for racing and wants to have a ss just like dad.

He's riding a 24" bike and I would like to know which gearing will a 9 y/o kid will be able to ride confortably.

Don't have a lot of cogs around to test with, just have a 18, 20, 22 to test. Will the granny and the 18 be a good combo to get started?