In the parents' profiles, I list the child in their birth order with numbers if the birth order is known or I can take a reasonable guess at it. However, in the children's profiles, I generally don't say what number child they were. If it's relevant to some genealogical point discussed in the profile, I would of course discuss it there. But if not relevant to some genealogical point discussed in the profile, then I think mentioning it in the child's profile is very much optional. I would definitely not always do it, since there are situations where we are not confident what the birth order was or how many siblings a person had.