I have a 3 year old child that has swollen tonsils and blisters on her tongue. What is this caused by?

Probably viral. Blisters on the tounge with swollen tonsils is most often caused by a virus. There are other cause possible. Sometime strep can cause the swollen tonsil while the tounge could be due to a vrus at the same time. Might want to see your pediatrician.