Your kids off ice trainer should be fired. But, maybe check with your kids Doctor and not the cesspool that is the Hockeydboard. I mean if your looking for nutritional advice here and not the Drs office you might be as clueless as the trainer.
Adding weight is done in 1 place....the kitchen! Add a PB and J (or 2) at dinner and a Milkshake before bed. Its not going to happen overnight but you will see the scale go up over the course of a month or 2. Until a kids body is mature enough to process the synthetic form of protein it just ends up in the toilet (Literally). Creatine is naturally made in the body so most adults 18-45 only need a few grams per day if any at all. Again, a kid will just get a sick stomach and it will end up in the toilet.
I’m a little surprised that the trainer didn’t talk diet first. Low calorie protein powder to help improve a kids diet would have been my first thought. Body type is important to, but if the guy is recommending creatine the kid must be skinny, so for example if the kid is 100 pounds then to slowly put on weight with some off ice conditioning with maybe some extra pull ups and push ups through the week they should be getting 100 grams of protein spread out over 5-6 meals/snacks. A scoop of protein powder with say 17 grams of protein per scoop with a tablespoon of peanut butter would equal about 23 grams total, so it’s not that hard to get to.
Obviously kids eating habits blow so you can’t get to strict at that age and make it a job for them but you can help them, 3 egg scrambled with some cheese and maybe veggies is almost 20 grams, shake 23, can of tuna at lunch is 25, ground beef or turkey with rice and spinach’s dinner is 30, and a shake after practice is 23, total over a 100 grams. The child’s body type depends on carbs/fats, a real active hyper non video game kid needs more carbs, a couch potato needs less. After the food intake and off ice training is religiously done for 6 months then maybe add creatine, don’t start with it.
The problem has always been with creatine the amount of water you need to ingest just to keep from developing some serious kidney problems when you take the stuff (look it up). I'm not too sure I would trust a kid to to pick-up his dirty socks as much as managing to drink gallons of water everyday.
Years ago there was some study done in Texas that found something like 80% of D-I high school football players were juicing. I'm sure it's alive and well in hockey here in N.E. The point being maybe this helps to open a pandora's box if you start them so early. I'd say at least nothing from the standard diet until the kid's balls drop.