can I use （女）孩子 even if the person in question is an adult?
as a chinese.I can tell you,yes you can use the word 孩子 to describe their duaghter,like 您的孩子/您们的孩子,or use the word 子女,it is more official,no matter their son or duaghter is or not a adult.or if you want to point out the sex,you can use the word 女儿 to describe their duaghter,or more literarily,use the word 千金,as you can describe their son with 儿子 or the literarily word 公子.if we use 公子 or 千金,we dont say 您的公子/您的千金,we say 您家公子/您家千金.
but we dont use 女孩子 to describe one's 40 years old duaghter,if she is about 20 to 25 and still have not been a mom,you can use 女孩子 to describe her.cause in our culture,if you have or had a son or duaghter you are not a kid anymore.and if you have not had a 孩子 and you are young,you may still be a kid in your parents' opinion.young women like to be called as 女孩子 cuase it sounds they younger.but just young women.I dont think 40 is young,so if you use 女孩子,it may sounds like you are satirizing.
my english is not good.hope you can understand what I write and hope this help