"My son will be 1 next week. My husband and I both work full time, and he spends three days a week with his grandmother, and two days with a neighbor. From as early as 5 months old, when he first started going to these respective places, he has consistently eaten more, a lot more, at the neighbor's house. When I was still pumping, for example, when he was at the age where I would take 12 ounces to my mother-in-law, I would take 18 to the neighbor. Six ounces to my mother-in-law, 12 to the neighbor. As you can imagine, this was serious supply problem when I was pumping. However, we got through it, and now I no longer pump. He has a little bit of drinkable yogurt or soy milk in a bottle or sippy cup during the day, but he mostly eats finger food.
Now that we are no longer under the constraints of what I can pump, the neighbor seems to be feeding him even more! On a normal, 10 hour day, I will bring her 9 ounces of soy milk, six ounces of drinkable yogurt, a banana, an avocado, and leftover dinner, such as pasta and sauce. It is rare that I would get anything back. However, I would bring my mother-in-law 3 ounces of yogurt, 6 ounces of milk, a banana, and either an avocado or dinner. Often she doesn't feed him the yogurt, and I will probably get either half the avocado or half of the dinner back. I know they both give him some of the food they have in their house, but for both of them, I believe that is incidental.
I am concerned about this, and I'm not sure if I should be. Can a one year old overeat? The neighbor argues, and she has a point, that you certainly can't make a baby eat, so he must be hungry. I think,
however, that if you offer him less, and then let him out of the highchair, he would be more likely to go off and play than want more, but if you keep him penned up, he'll eat what you give him next just as something to do. It's hard for me to judge his appetite myself, because on weekends he still nurses a great deal, and in the evenings he has a bit of cut up dinner, and then nurses. He is only starting to be night-weaned (Sleeping issues are a whole other post!).
Should I just let this go because it's only two days a week? Am I setting him up for bad habits?"
Bad habits? No. I think what you're teaching him is that the rules are different with different people. That means he's going to be able to roll with it more easily as he grows older. So I don't necessarily think it's a bad thing to learn.
It seems like there could be any number of things going on here. One possibility is that the neighbor really is encouraging him to eat more, but you can't actually force a baby to eat. (You can push bottles when a baby's only drinking, but I can't imagine being able to force a baby to eat solids.) He could be just eating more out of habit while he's in the highchair, as you guess, but if the food is nutritious it's not a real problem. Another possibility is that your mother-in-law is interacting with him way more and he's too busy to eat at her house, so normally he'd eat an amount closer to what he eats at the neighbor's, but he's too high on life at grandma's to eat much there. A third possibility is that your MIL is feeding him a lot of other food and not telling you, so his intake is actually closer to equal in both locations, but you only know about the food you bring her. (Only you know if that's a real possibility or not.)
Of course, this is really all academic, because I don't think there's much you can do about it, and now that you're past the pumping stage it isn't really an inconvenience for anyone. If he's growing and happy and you're not spending down his college fund on drinkable yogurt*, I think your life will be simplified if you try to let it go. In another few months he'll probably go through the first of many picky stages, and you'll be wishing he'd eat something, anything that isn't white (or green, or orange, or whatever color he goes with). So it's good that he's packing it on now, like a squirrel storing up nuts for the winter.
If there is some kind of vast food conspiracy of any sort going on, you'll probably find out about it in about 15 years when your son brings it up in casual conversation. "Mom, remember back when I was little and you thought I was eating too much food at the neighbor's? Well,..."
Have a good time at his birthday party. It's hard to believe how fast time flies, isn't it?
* If you want to save money on drinkable yogurt, you can buy regular yogurt and mix it with milk to the right consistency, and voila!, drinkable yogurt at half the price.