Are you able to make Peter Pan your last ride of the night? As long as you all are in line before park closing, the CMs will allow you to ride. It can be magical to exit the ride to a closed park, then take your time going down Main Street (look in windows, shop a little -- stores are open an hour after closing, take lots of photos), look back at Town Square, MS, and the castle before exiting the park.
As for the motion sickness, lots of us suffer from it to varying degrees and still manage to ride a fair amount of rides. Do a search on this forum for threads on motion sickness. There are some very good suggestions. If you find something that will work for you, even a little bit, great! And even if you don't, you'll still have a wonderful time at DLR. There is so much to the parks beyond rides.
Also, download the free DLR app so that you can be monitering wait times before you enter the park. Have a few choices for rides so that you don't get stuck in a long line on your first ride ever at DL. There is also an information kiosk by the entrance to Adventureland that has a ride board with listed wait times. You might change your choice based on the information at the time you enter.