Can you explain if it's possible for my 6 year old to have lymphoma and neuroblastoma at the same time?

Unlikely. It is unlikely but still possible. Both lymphoma and neuroblastoma may be sometimes characterized as "malignant small blue cell tumors" so they may look similar under the microscope and special studies are necessary to differentiate one from the other. It is however possible for them to occur simultaneously ("collision" tumor) but this situation is very rare.