hi
According to me the chances is more of restoration/filling which is done is high, because child won't be able to judge the high point in the restoration during the procedure, you have to visit the
dentist and get it corrected.
Secondly, it can be due to RCT procedure is not done properly, the
root canal filling material is either short in tha canal or flown out of the canal.
Ask your child not to eat or chew from that side, till its corrected.
Give the child lukewarm saline mouth rinse 3-4 times/daily.
Give the child
soft diet.
Don't give hot compression from outside.
Don't let the child sleep from the side where is swelling.
If the child has unbearable pain, give a pain killer tablet.
Swelling is temporary it will go away, not to worry.