If you are providing daycare, you need to have a bag just like a daycare center would. That means a change of clothes, a snack and a drink. Your DIL should know what her child needs to eat during the day and provide it. He must eat lunch at home, and have drinks and snacks. You shouldn't have to tell her what she needs to bring for food.
That being said...an 18 month old being in jammies all day isn't that bad. If they are clean, if there is a change of clothes, if there are diapers. He is still a baby. In the winter he should have a coat and shoes, and when the weather warms up he will need outdoor play clothes. This might be a misunderstanding, at least in part. She knows you have clothes for the kiddo, so she doesn't bring a change. You said you would provide food, so she hasn't offered again.
It sounds like your parents were particularly uptight about clothes, and you are less so. If the worse things the kid encounters in life are all day jammies, and uncombed hair then life is pretty good. If the kids are happy, well cared for and loved, the appearances just aren't that important.
It might be that you all need to sit down and discuss what eaches expectations are, and responsiblities. It sounds like you have different values and priorites and talking about them calmly and nonjudgmentally will help clear the air. It sounds like you're building up a pile of resentment that might hurt your relationship.