I know I can’t take away their loss, but what can I do?

Condolences don't always have to come through written or spoken words. There are many things loved ones and friends can do to remember the child who has died while still expressing their grief toward the family:
Listen.
Have a tree, flowers or bush planted in memory of the child.
Just give them a hug; no words are needed.
Don't remove pictures, artwork, etc. from your house for fear it may upset the bereaved parents.
If the deceased child had siblings, offer to take them out to a movie or to the park. They're grieving too and need attention.
Place flowers or another memento at the child's grave site, if applicable.
Make a monetary donation in the child's name to a local charity or hospital.
Send the parents some flowers or a plant.
Make a few meals in freezable containers and leave them at their house.
Hire a housekeeper/gardener to come in and take care of things for awhile.
Do not remove any of the child's stuff from the parents' house.
Just hold the parents' hands and allow them to cry or talk about their child.
If you live out of town, periodically send a postcard to family, just to let them know you are thinking about them.
Offer to scrapbook items about the child including birth and death certificates, obituary, locks of hair, hand prints and other cherished items.