If your child ranks in the 5% category for height/weight it probably means his or her parents are of slight build as well. And if your child tells you he wants to play nose guard on the football team or go out for basketball you would likely try to steer them in another direction. Although their are exceptions size and weight matters in these sport- you can see it matter. There is no point in encouraging them to put their hopes and dreams into a sport they will not be successful in. When it comes to speed however parents often miss that it is genetic as well, although much more trainable. Distance runners tend to be small, very lean, ectomorphic, and slow twitch. Sprinters tend to be more muscular, lean, and fast twitch. But there are few runners competitive at 260 pounds. Large, endomorphic individuals with more body fat may be able to move for very short distances but generally lack sustainable speed or endurance. Yes, there are exceptions but you are generally trying to cram a round peg into a square hole.
There are numerous genetic components to being a good runner beyond just body type. These include capillary density, mitochondrial density, size or hear and lungs (VO2 Max), tendon insertion point, muscle makeup, and the ability to thermoregulate to name just a few. You can't see these things but they are very important, and in ball sports like soccer and lacrosse they are critical. These sports require running speed AND endurance as pre-requisites to getting to the ball or getting open. There is a lot of emphasis on skills in these sports but they only matter if you can get to the ball and utilize them.
As your child progresses in these sports they must be able to keep up, and keeping up becomes more important as they progress. Parents can become frustrated by a skilled child that is being left behind on the field. Some of this may be due to under-conditioning but if they are giving their all at practice and still falling behind it is important to understand that some of it, or a lot of it, may simply be genetics. This becomes more apparent and more relevant as the child enters the teen age years and sports like soccer and lacrosse become more competitive. Your child may in fact be giving it their all they just don't have inherent speed; just like they may not have inherent size or height. Be aware of this before you pressure them to produce more speed they may not be capable of producing.