Hello cricketke as you probably know bilateral means both sides.
Pars Interarticularis is a portion of the bone at the rear of the vertebrae that is between the top, and bottom of the two facet joints that guide motion between the vertabra. So they are talking about this part on the left and the right.
I believe the condition is when there is a separation or fracture through the Pars Interarticularis, dividing the main vertebral body at the front from structures at the rear of the vertebra.
Its most commonly seen at L5 the bottom vertebra, and can result in spondylolysis, which is when one vertabra slips forward in relation to the other vertebra, i think you should restrict her activity for now and ask the ortho about it.
all the best