The aim of the trial is to describe the safety and efficacy of intravenous (i.v.) Treosulfan compared to the conventional (myeloablative) dose of i.v. Busulfan, each administered as part of a standardised Fludarabine-containing conditioning regimen and to contribute to a PK model which permits - in conjunction with data comparing Treosulfan and Busulfan in adults with malignant diseases - to extend the use of Treosulfan in the paediatric population by extrapolating efficacy.