for pediatric heme/onc fellowships, there are approximately the same amount of people applying as there are positions. so, if you want to do the fellowship, there should not be a problem getting a spot (90% of US grads matched). however, it is fairly competitive to get a spot at some of the large, well known, academic centers (mskcc, dana farber, chop, etc). below is a link for match statistics from this past year's match.
For most fellowships, the first year is mainly clinically oriented in the hospital and the last two years of the fellowship are research oriented.