Extra Carbs for Exercise?

My 8-year-old daughter, who was diagnosed 3 months ago, is on a swim team. Each January, she swims in a one-hour “marathon.” She will swim hard continuously for a total mileage of about 1 and 1/4 miles. Does anyone have any advice for me about how many additional carbs to give her for this intensity of exercise? When should I give them to her? Before, during and after the event? Is it important to cut back on the normal insulin she might have in her system (like from an earlier meal) in anticipation of the event?

I had her do a 30-minute practice swim. With a BSG of 132, I gave her 20 additional carbs and no insulin (but she still had insulin in her from lunch). She got out of the pool at 55.

I would appreciate any input you have about dosing for exercise. The Dex-Com stops transmitting immediately when she gets in the pool, and she does not want to stop to test her blood. She is really committed to this event, and we are trying to give her access to her pre-diagnosis activities, but I’m finding this situation challenging. Even swim practice is challenging to dose…I still have a lot to learn! Thanks for your help.