J.M.
For the baby I would suggest something personalized. With all the hand me downs she will be getting it might be nice to have a bank or a pink blanket with her name on it. Also, a girly outfit might be well appreciated especially if they had a boy before.
For the boy I would suggest anything that you think he might like would be great for him. He will enjoy getting some attention to no matter what. But maybe you can find a book about a brother and a sister or about what it means to be a big brother. Something that will let him know how important he is going to be to the new baby.