“Is covid-19 dangerous to a child who recently had radiation?”

It can be, yes. A certain amount of immunosuppression is associated with radiation therapy, but that depends on the anatomic location and the therapy dose/regimen. However, please know that the survival statistics for children and even adults under 70 are vastly favorable in all comers....over 98%. Zinc and pepcid may be protective against COVID. I currently take them if that helps...
All the best.

Radiation therapy, particularly if it involves a significant portion of the skeleton, and therefore bone marrow can lower the immune system activity and make people of any age more susceptible to infections of any kind. This would be a good question for the treating radiation oncologist and your physician since there are several follow up questions I have regarding
your child's particular case. They would immediately know the answers.

There should be no added risk due to a recent radiation exposure.