Strength and speed training for U11s....thoughts?!

Way are people's thoughts on 10/11 year olds doing strength/speed training?

I'm guessing that many will disagree with it and say that kids should be learning skills, getting match practice and enjoying themselves, rather than getting bogged down with this kind of training. I'm also guessing that there's a school of thought that children are too physically immature to be putting extra strain on joints etc.

If there's anyone out there that knows of any evidence for, or against, I'd be really grateful. I coach a team with a number of (in my opinion!) quite talented players. At least one of them is somewhat let down by his lack of speed and a lack of strength.

Should I let them concentrate on learning, and leave the speed/strength training until they're a few years older, or does anyone have any advice on what they could be doing now?

Thanks!