Pro tip: Stop making such fattening food. If a single serving is more than what will safely fit in a child's stomach, macro-eat will prevent child abuse.
Actually I've had the same problem as Count. They've never been able to macro->eat unless there is prepared food available on the lot. I
think that if I time it so that an adult is already laying out food during their macro->eat, then the kid can nab it, but otherwise no inventory food for them. I ran tests in the past to verify that the food itself was fine, but don't remember the details clearly at this point. Well, beyond "Yep, they should be able to eat this," that is. I'm unsure if it's a factor in context, but I don't have FEY yet.
New test with gelatin, as it's notoriously not filling.
1. Child with ~1/3 hunger bar. Had an adult serve Gelatin.
2. Put both the serving platter and a single plate into the child's inventory: no macro->eat.
3. Put plate on the floor: macro->eat became available.
4. Used it and the child ate the gelatin on the floor. When done, the child's hunger bar was at ~2/3.
Conclusion: it doesn't seem to be an issue of overly filling food.
ATM, I just toss some food on the floor when I notice the kids getting hungry, and let them have at it with macro->eat. It's a hassle, but it hadn't gotten to the point where I could focus on getting an answer rather than getting distracted by all the tutorials, fixes, and shinies. Which, incidentally, was how I found this.

Today, though, I intended to tell an adult to Macro-Garden for an adult, but didn't realize that I still had a kid selected (there are seven sims in the house now, heh.) Macro-Eat was present for the child--despite the fact that her hunger bar was completely full and green. However, when I let her go down to about an eighth-full, Macro-Eat was not present. One adult and one teen had food in inventory; grilled cheese and chef salad, repectively.
Huh, weird. Perhaps the kid was just shy of full and there was a mostly finished plate somewhere? That or the kid has a bizzaro!stomach. "Me not hungry! Me
very not hungry! *keels over*"