Kids bike rear derailleur change I am not too technical when it comes to bikes, but I wonder about the change of rear derailleur.

I bought my daughter a second hand good shape bike for good money Cube Acid 240 disk with Altus 7 speed derailleur. I wonder if it would be possible to change the rear derailleur from 7 speed to 11 speed SLX? I wonder if it is possible, and if yes how much do you think it would cost (I assume changes will need to be made not just the derailleur but the more things on the bike) and whether it makes any sense?

Thank you in advance.
An upgrade to the drivetrain on this bike is very likely not possible.

A 7 speed drivetrain either has a old-style freewheel, where the sprocket cluster and ratchet mechanism are in a single unit that threads on the hub. These can't be upgraded to more sprockets.

The hub may have a newer style freehub design where cassette sprockets slide on a splined cylinder, but 7 speed freehubs are narrower than those designed for 8 speeds or more. 

The wheel might have a 8+ speed freehub and a 7 speed cassette installed with a spacer, but this is unlikely.

When answering questions about upgrading the drivetrain, most of the time my advice is that's it's not worth the cost. Separate drivetrain components are expensive, changes in one place have a knock-on affect that require other components to be replaced. Sometimes the entire drivetrain including shifters, crank and rear wheel need to be replaced (although not in your case). Also, you don't get the full benefit of better quality components when they are installed on a lower quality frame.

Personally, as this is a kids 24" wheel bike I'd leave it alone and save the money for your daughter's next bike.