During the Phase I portion of this study, when you enroll, you will be given an assigned dose of ABT-751. The dose of ABT-751 will be based on doses given in previous studies done with adults and children. At each dose level of ABT-751, between 3 and 6 children will receive ABT-751 in combination with chemotherapy. If the side effects are not too severe, the next group of children will receive a higher dose. The dose will continue to be increased until we find the dose that causes serious side effects. Your dose of ABT-751 will not be increased. If you have bad side effects, your dose may be decreased.