There are full length goalie pants, to 3/4 sized goalkeeping pants, so this makes it a very difficult choice to make sure you pick the correct pant.
I have been coaching for over 20 years now, I have coached close to 10,000 goalkeepers in many many countries with my GK Coaching academy and I would say as many as 95% of parents get this wrong, which is a worry.
Goalkeeping is a very strange and wonderful position and any experienced goalkeeper or goalkeeper coach will tell you there is a lot of mental focus on playing in goal. And if a goalkeeper does not feel 1000% comfortable in what they are wearing they WILL NOT play to the best of their abilities.
Even if its a young goalkeeper and who will not understand the importance of the mental side of goalkeeping, subconsciously if they do not feel comfortable with their goalkeeping garments whether its trousers or goalkeeper gloves, they will not move correctly and thus not play to the best of their abilities.
I am sorry to frighten you as a parent but it is really important that you buy the correct goalkeeping trousers for your child.
What also makes this more difficult you cannot really by quality goalkeeper pants in shops now a days, it seems to be the cheap and nasty stuff for sale, which is normally no good. The only place to get professional goalkeeping trousers in online but then you have a bigger problem… how can you buy before you actually know what fit and materials you are buying.
There is no easy answer to this because it is a matter of trial and error before you get the pair that will best suit your child and is the correct fit. So it does mean you may have to try a few brands before you find a brand you can trust to not only sell you good goalkeeper pants but to also offer yo free help and advice.