Go to churches in your community. Some have a "giving tree", whereby members of their congregation take a paper ornament (keeping you and your grandchildren anomonous) with a gift idea for that child. A gift is purchased, wrapped, and delivered to your family. Many Department stores, such as Walmart, have done this too.
You will have to get your information to whom ever is coordinating it, soon, so you might start inquiring now. You are not alone in needing this type of assistance. The church I belong to has a giving tree for families local to our community. We also provide food assistance, with a food pantry housed in our church. You should be able to find some local resources, if you look for them.
The gifts received, may not be extravagent--but it may help with some basic needs. However, you may be surprised by the generosity of strangers!