Hi, Im sorry to hear about your child. Floaters usually occur in individuals who are above the age of 40, mostly above 60 years. They can be a sign of retinal detachment however considering that you have already had your child checked by a opthamologist who has cleared him or her of any retinal problems or even vitreous humor issues, its less likely that they are associated with the eyes. Now coming to your question, can this be a sign of any brain tumor? Well in certain cases, tumor can present in this manner however they will usually be accompanied by other symptoms such as headaches, vomiting, weakness, visual disturbances such as vision loss, double vision, balance issues or even behavioural changes. If your child is experiencing any of these symptoms alongwith the floaters then yes he or she should definitely undergo an MRI brain to rule this out. In the absence of any other symptoms, the decision to get an MRI done will be based on the findings of the clinical exam specifically the neurological examination. The patient here will have to undergo a visual field assessment as well because many children can have tumors which interfere with the visual fields. Comprehensive examination of the visual fields usually gives a pretty good idea as to the presence or absence of such a tumor.
It would be very difficult to determine whether an MRI should or shouldnt be done just on the basis of floaters. I would recommend that you have your child examined comprhensively from a pediatric neurologist with emphasis on the visual field assessment alongwith assesment of other neurological function and take a decision then.
Another scenario would be if the patients symptoms that is floaters are progressively increasing in frequency and number over time, then obviously a MRI should be done to determine if this is the cause.
I hope this answers your question, please feel free to ask any followup questions. If i have been helpful , please accept my answer and leave positive feedback.