Ridiculous in the first place to encourage children to ignore their symptoms. If their cancer returns, and they continue to follow the LP process of denying symptoms, early warning signs may be missed.
If these children become ill with something else, following the LP process, would they deny this as well? Where does this stop?
Since CBT and other psychological treatments are considered safe by most people perhaps the assumption is made (without any proof) that LP is equally "safe".
Around a third of my job is the diagnosis, response/complication evaluation and long term surveillance of infants, children, adolescents and young adults with cancer. Reporting change in symptoms is very important for clinical follow-up. There are also objective signals to alert for early/late recurrence: blood markers that can be followed in some solid cancers and imaging surveillance (my area).
At presentation, it is a common situation that the tumour is very large (in a patient that is very small). Think of an iceberg. The thought that it could have been found earlier adds to parental distress. We reassure them that this is a typical feature in paediatric cancer diagnosis and parents will usually indicate a resolve to be very vigilant in the future. This is not in any way pathological - it is very helpful. Unless very young or otherwise unable, the child/young person is also encouraged to talk about changes in their body over time.
Interval surveillance does have a small risk of disease developing between assessments. For the primary disease, local recurrence and distant metastases are the usual concern, but as DokaGirl indicates, other diseases (atypical infections, treatment complications, second malignancies) can and do occur.
Beyond the normal human sensitivity you would expect when celebrating good or breaking bad news, I do not have any special psychological training/insight. However, I would be very concerned that the patient reporting safety net and the therapeutic relationship itself could be negatively affected by such a psychological manoeuvre.
Assuming I can get back to work soon, I will ask my oncology medical and nursing colleagues for their views on this. I would be very surprised if they were supportive.