Hi Sara!
Welcome to planDisney! I'm sorry to hear that your little one is under the weather. I know dealing with any illness while on vacation is never the desired outcome, but I'm sure you are making the most out of your time so far. I hope the beginning of the trip was as magical as you hoped for!
Typically, park tickets are only valid for the dates that were stated upon purchase. Based on the information you provided, it seems like your tickets are only valid until March 8th. Unfortunately, you can not make any changes online using your My Disney Experience account past your allotted date range once the tickets have already been used. However, I would
call the Existing Tickets line and speak to a Cast Member for further assistance. You can explain your situation to them and see if there is anything they can do on their part. If you are staying at a Disney Resorts Hotel, you can also stop by the front desk and speak to a Cast Member there. While there is no guarantee they can make any adjustments, it's always a great idea to let them know what is currently going on.
Sara, my family had a similar situation arise on a recent trip we took to
Walt Disney World. My daughter was sick on our second day of the trip and didn't bounce back until a couple of days later. While this can put a damper on the vacation, the Cast Members at our resort were all extremely wonderful and we made the most out of our lost time. We played games in our room, watched endless Disney movies, and even had a few Mickey-shaped treats while recovering. I would highly suggest reaching out to see if any accommodations can be made.
I hope your child is feeling better and you have an amazing end to your stay, Sara!
Have a magical day!
Greg