Children Can't Take Medicine?

does the sim have a moodlet?

Children can only take medicine while they have an illness moodlet. If he doesn’t have one, then try having him take it again next time a moodlet shows up.

You could also make the child drink orange juice to cure their illness.

He didn't have a Moodlet at the time, but he was showing symptoms (acting dizzy and stars swirling around his head. No clue why no moodlet showed up then.) Later, I had him drink orange juice, he got a home remedy Moodlet but then showed symptoms again shortly after. Then he had the illness Moodlet and I was able to have him take Medicine. Strange sequence of events that don't really make sense. Also, if they're sick, why doesn't the Moodlet last the whole sickness? It disappears so fast it's easy to miss it. The Moodlet should stay until the illness is cured. It just makes sense.

Is there no way to curb all this illness nonsense? I know it's Winter, but enough is too much already. After I cured the boy, his mother got some itchy disease, his big brother got a rash, his sister got a fever, and now the boy is taking yet another sick day already. This is making me completely mental. Oh, and the cat is sick now too and I have to take it to the vet. Madness! Utter madness!

If you have testingcheats enabled, you can shift-click a Sim and select the illness menu; there’s a “Remove Illness” option in there (as well as “Remove Symptom”, which has to be done first if there’s any symptom moodlet present). I’m not 100% sure it’s possible without a cheats extension mod (like UI Cheat Extension or MC Cheats), though.

I'm guessing it needs the mod. I've never seen an Illness menu and I Shift+Click my Sims frequently. Sometimes to debug a stuck animation as well as the Needs cheats. Disable Need Decay seems to come undone every time they travel, especially for anyone left behind. Since I can't see my pets' Moodlets, the Make Happy cheat is the first thing I try when I see they're out of sorts. I'm sure I would have seen it by now if it was there.

Do you have “Change Sim Info... > Remove Moodlets” in your shift-click menu? If it’s there, you can use that to remove the hidden illness moodlet. It’s the one with the name of the illness (e.g., Llama Flu).

No. Change Sim Info isn't in the menu either. Now you mention it, though, I do have this cheat... sims.remove_all_buffs
Maybe that will work. Then again... maybe they'll just still be sick without a corresponding Moodlet. I've had something similar happen with this cheat. I can always try it I suppose.

That cheat used to work, but it got removed with the latest patch.