Changing the size of crochet bath robe

I have a bath robe pattern for child size 2, 4 & 6. I want to make it into a 8, 10 & 12. Any ideas? I tried to just add some stitches, but it didn’t come out right. I have an aunt who would like me to make the robe for her. Can a child’s pattern be made into adult? Any help will be appreciated. Thanks, Joanne:yay: :happydance: :woohoo: