How can I work out what hook/yarn size to use?

[yarncakes]

I want to make dungarees but when I find tutorials online, they're all for babies/toddlers, but I want to make a pair for a child. How can I determine what size hook and yarn to use to follow the pattern? For example the pattern for 0-3M calls for 2.5mm crochet hook and DK yarn, but how do I know whether 5mm and Aran yarn would fit a child? If that makes sense? I hope someone can help me out or simplify it a bit more for me! Thanks in advance xx

Ordinarily you'd use the same hook and yarn but do more stitches to make the item bigger. I don't think substituting Aran will work.