Close-up shots of Kids

I've learned that kids are fast...really fast especially when it comes to taking pictures of them.  I think I took at least 20 pictures of nephew "S" above and the best one was this one.  Mastering the art of capturing a child in the momemt is one of my goals as a photographer because they're only small once and they change interests so quickly. How fun would it be to remember how much your kid loved trains and obessively played everyday until they got involved in trucks?  I want to capture those moments. :)