Here are some ideas to get you started...
Offer to go on a doctor appointment and/or babysit the siblings. Children with a diagnosis often have extra doctor appointments, sometimes scheduled up to a year in advance. These appointments might be 30 minutes away or at some specialist 3 hours away. Offer to go along and be an aide while mom fills out paperwork or asks the doctor questions. Alternately, mom might need a babysitter for her other children.
Make a busy pack. This so easy, inexpensive, and even a little fun. Find a craft of the appropriate level, assemble instructions and materials in a zipper seal bag, and hand it to mom. If you are not the crafty type, find a few inexpensive toys, maybe a small book, and a novelty treat. Give it to mom on the sly so she can add it to her arsenal to subvert a tantrum or rainy day meltdown.
Offer to help at church. This is a big one. Church should be a refuge, but for special needs parents it is too often a depleting war zone. I can't begin to tell you how many parents would be encouraged by someone asking if they could help with your child during Sunday School or take a child out to the playground after church so the parents can enjoy some fellowship after the service. It would absolutely be a gift.
Donate an hour.- Offer to run an errand, take their car to the wash, or take the kids to the park for a nature walk. How about an hour of respite in your home? It might just be the time a parent needs to make phone calls, do paperwork, or just decompress. We'll take it.
Give your special skill. Photographer? Offer a free mini session. Baker? Drop off a couple loaves of bread. Do you enjoy sewing? Take on the mending pile. It all makes a difference.
There are probably at least a dozen more ideas, but this could serve as a jump off point if you choose to bless a parent of a special needs child on any occasion. If you still find money to be the best option, no one in the history of mankind has ever turned down a gift card. Also, many parents rely on services from non-profit organization that do yearly fund drives. Ask when the next fund drive is taking place and donate at that time.
Finally, sharing this list in some way would be a free gift, too. Who knows where it might touch a heart and inspire goodness?