Thanks for reporting back!
Tested again:
Uninstalled Into The Future EP (I am on Steam). When loading the save there were no message about 'missing content' like I have seen on other occasions when loading a game that was using content temporarily uninstalled.
When the child took a joyride, the previous ITF special outfit were now changed to a base game pajamas, colour light blue - not a default colour of it in CAS. The shoes (not from ITF) were the same as before. I tried to change the child's outfit in a wardrobe but there were no option to. ('Change appeareance' in a mirror worked and stayed as the new 'special outfit': tiger face make-up and plantsim flower bud hair instead of helmet). My game is vanilla /unmodded, and I could not change the child's outfit with testingcheatsenabled, the special outfit hindered it like you noticed.
With testingcheatsenabled (plastic surgery not being an option for children) I changed the child's skin to other colour: dark blue. When taking a joyride the skin was changed back to default one/the one the child had on their first joyride (so peach pink skin, light blue pajamas, flower hair and tiger face - funny).
I reinstalled ITF, but that did not change the child's special outfit (still peach pink skin, light blue pajamas, flower hair, tiger face).
My guess based on this is that the children in your game had a skin versus outfit on their first joyride that somehow got changed later on and that the CC (?) skin and outfit is still accessible for the game and therefore not exchanged to something else even when you uninstall everything.
I do not have CC so don't know that much about it or how to remove it properly, but here are some hopefully useful links:
User files in sims 3 explained on crinrict's blog, the 5 cashe files and DCCashe I think is mostly important in this case:
https://sims3.crinrict.com/en/2011/01/faq-user-files.html
How to remove CC explanation on AHQ, see especially "my sims are all ":
https://answers.ea.com/t5/The-Sims-3/HOW-TO-Find-and-remove-bad-custom-content-CC/td-p/2759940
I do not have the mod MasterController so could not try that, but if you are interested in it you will find it here:
https://www.nraas.net/community/MasterController
