Sentence 1 is correct. It means if the baby seems unable to understand it doesn't matter. Sentence 2. is correct also it means it doesn't matter if the baby seems unable to understand. "it doesn't matter if your baby seems TO NOT understand" you will hear this phrasing also. I am not sure how much grammar maybe involved or how much logic is involved the logic for any sentence like this most often gives the same end result. 1. does not x if y seems[apparently] not to be/doing X = [if y does not/is not apparently X ] 2. it does not matter if X seems[appparently] doesn't seem to be/doing Y=[IF Y does not apparently seem to be/doing x] Sentence 2 some may say has a double negative with the extra doesn't seem to, and other people may argue the double negative has been negated back to simply doesn't.

The second sentence is correct. You can use "not" in the following way: "It doesn't matter if your baby seems to NOT understand what you're trying to say." I hope this helps Andres!

1.) It doesn’t matter if your baby SEEMS TO NOT understand what you are trying to say. 2.) Is correct.