I understand. It sounds like you have an extended family in one home. From raising 4 children in a blended family, my kids went through food jags.
Sometimes it was just because they didn't like what we were having.
Sometimes it was a control thing or an attention seeking behavior. I
know it is difficult to change our parents, however if you could ask him to back off a little bit on both you and your daughter, she will start to come around with the eating.
A rule of thumb that you may or may not agree with is -when they get hungry they will eat. If your daughter just wants to eat PB&J, that is okay. Just make sure the ingredients are healthy. If she only wants to eat broccoli, that is okay too. Even my pickiest eater(now 18 years old) is very athletic, plays college soccer, and is a pre-med student. She was always picky but she did find that when she at least tried something sometimes she really liked it.
At our house, we have a table/meal rule - if there is something on the table to the meal that they have not tried, they at least ought to try it to see if they really do or don't like it.
I would say if there is that much negativity in the home, your daughter is using this as a control mechanism and the negativity may be making it difficult for her to even get relaxed enough to eat.
I would imagine it is difficult for you to eat sometimes with the atmosphere. Maybe she could eat after grandpa has left the table. My in-laws always had a fit when I did this but if the atmosphere was such my kid couldn't eat, having them eat in an area or at the table without all the commotion helped them calm down and eat.
As long as she is growing on her growth curve, looks healthy, plays, and overall looks great, I would try to calm the atmosphere at home, let her have her food jags for a while, and even get her interested in helping to cook like you have been doing. You could also give her a children's vitamin each day just to help ensure she gets the right vitamins and minerals. You might also try an instant breakfast drink and see if she will drink that when she doesn't want to eat. My youngest would drink Pediasure when he wouldn't eat otherwise. The instant drink mix is less expensive.
Hang in there,Customer You are good mother.
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