The rationale for this phase-1 trial is to find an appropriate dose of omega-3 for future phase-2 studies. The investigators aim at defining a dose that can safely be given to children previously treated for cancer, and that does not cause unacceptable side effects. In a longer perspective, randomized control trials are planned where the supplement is given for longer periods of time, and in combination with conventional cancer treatment. In preparation for these trials, this dose finding study was designed that also evaluates compliance. Subsequently the investigators hope that this supplementation can be beneficial for children during both cancer treatment and after remission.