Well, I say the honor system is in play here. I'm sure the couple is not going to check up on their guests to ensure that everyone has in fact made a donation. In fact, because they are older, I'm sure they would understand that you are still in a phase of your life where extra income isn't common (as a SAHM of 2). So, cut yourself some slack - if you can make the donation, do, but do it for you (in the name of the brides mother) not for them. You know? Also, I wouldn't show up to the wedding or reception with anything in hand to give them and this holds true even if I had been able to make a huge donation in advance. It would be my assumption that by asking you to donate directly to a charity they are asking for no gifts. When my husband and I got married, we asked for NO gifts. We made this clear to everyone who was invited. We had already established our household and didn't need anything (as is the case for your couple, I'm sure). But we had a few who just thought we were being gracious, I guess, because they ignored our wishes and brought gifts. Well, they ended up feeling awkward because we had not set up a table or any designated area for gifts. Nor did we make time in our planning of the big day to give/recieve gifts. So, those who did have gifts just held onto them awkwardly at their tables until the end. By the time they were able to give them, it was just all very awkward and I have a feeling they wish they had listened. So, listen to the couple in your life. They said no gifts. They have asked that anyone wanting to get a gift anyway, donate and you plan to do that. So, when you can, do. I see it as simple as that, but I've probably just informed you to make half a dozen wedding faux pas! ;)
Have fun at the wedding, okay? (No scanning the room for cards from other guests!) :D