I thought about buying an Ipad 10.2 inch and wanted to ask if you could play on the iPad Fortnite in 120fps
No do not go
No you can't.
You will never be able to play 120fps on mobile devices, but the real question is: why do you play such a bad game?
Buying a PC is more useful than unnecessary Apple devices.
Fortnite in 120fps you will not be able to play the thing does not even have 120hz, that means 120fps bring you nothing.
And on PC there are much better games than Fortnite, like Half Life 2, Portal 2, Forza Horizon 4,
shellshock live and much more
Got a gaming pc and the tablet is for my son who is 11
If I could not have known you are talking about yourself.
Can't he play on your PC?
An iPad is not as good for gaming as a PC or a console, the resolution is poorer and frame rates are low and the battery will quickly run out.
I would buy him a Nintendo switch. Or your own PC? After all, he can do more than on a console or an iPad. A switch would be most suitable, but you have to make the decision.
But on the Ipad Pro 2018
If you have no idea maybe. Keep one's mouth shut!