The patterns are accurate ... according to the standards of the company making them!
Just like us ... you can go to one store and a Small fits, then to a different store, and a Small is way to small! Or what really gets me is that in the same store, one manufacturer's Small works ... and the other one, you need a Large.
So the only way you'll know ............ Measure child ... Measure pattern.
Compare and Decide.
You can always lengthen it when cutting out the pieces.
Check that the waist falls where it should be, or you'll be needing to lengthen that section too.
If the pattern size works for the child, great.
If not, better to purchase a new one, even if you have to absorb the extra cost, than to fiddle too much with altering the existing one.