In my career I have sized up approximately U8s 4,500 times. More or less. The ratio was approx this -
1,500 out of a box starter kits in Youth sizes
3,000 kids one piece at a time
From fitting properly, we were able to get most of those 3,000 kids into properly fitted gear. Some parents were insistent in buying something too big "so they can grow in it" , or they came in in May when I haven't received a pair of youth medium elbow pads in 3 months and we have to go with a large. Of those 1,500 kids who got sold a starter kit, I probably changed sizes over 200 times. Usually because the kid is too fat. But some of the brands have been very awful in scaling of sizes, ie elbow pads too big compared to the shoulder pad size, or shin guards with straps too short, or pants too big, etc. The others I fitted I doubt half of them fit great for every piece, I am positive a lot of mis-sized gear is out there with it.
The core issue of those starter kits is in order for them to be financially feasible for the LTP market & retailer to have a "kit", is this is made as cheaply as possible in order to make margins, as it is razor thin at this level of equipment. This gear therefore has faults such as sizing inconsistencies or issues with the product itself being defective. It does accomplish its goal though - it gets more kids into hockey by introducing players at a lower barrier. It serves a valuable purpose that cannot be overlooked.
Stating that, I have yet to find a full set of youth kits that are proportionate w/ good quality at that right price. There are better kits than others, however, if you can afford it, my recommendation is to size each piece of equipment individually. A lot of LHS do deals as in "Buy X amount of youth gear get % off" or something similar. It rarely goes on sale but when it does, buy the next size up of what your player has right now if they like it. Don't get greedy, just get the next size up on sale. Youth gear discounted is dirt cheap.