I think people are maybe being a bit more negative than they need to here! Sure, there could be potential problems, but there could also be benefits - I do exactly this for a neighbour (it started out as 1 morning a week for free, but then eventually went up to 4 mornings which she pays me for as she didn't feel comfortable with getting so much free childcare). I can honestly say that the advantages far outweigh the disadvantages.
Having somebody else coming for breakfast gives my family a real impetus to get out of bed and get dressed in the morning (which I know would otherwise be an issue), it means I'm more organised as there's no way I would ever let somebody else's child risk being late to school, and my dds have become really good friends with this little boy (he's actually a year older than dd1). Sure, we've had days when dd1 has been ill, but for us it was easy as the school is only round the corner, so I took the neighbour's child in anyway. We also had a morning when I was in hospital, but my husband took both children to school that day. Your neighbour will need to sort out some sort of back-up childcare anyway, whatever she ends up doing, so I wouldn't get too worried about emergencies - they're bound to happen occasionally, but generally speaking you're going to be taking your ds in anyway, so in the normal run of things it wouldn't make much difference to you having an extra child along.
I think people have raised some important issues here, but I don't really see why this should mean you should reject the whole idea immediately: if you did offer to do it, perhaps you should suggest that after a month or so you could have a drink together and talk about how it's going? Obviously if the two children didn't get on, that could be a pain, but on the other hand it's not a long time that they'd be together, and presumably most of that time would be spent having breakfast so fairly unlikely you'd have fights breaking out all the time.
I know you say you wouldn't be comfortable being paid - this is exactly how I felt to begin with - but bear in mind that she might feel uncomfortable not paying you for something which is obviously helping her out enormously and enabling her to go and earn money for herself! For me being paid definitely helps me not feel exploited/resentful, and there's no issue with being an illegal childminder if it's just the school run in the morning (in my case I actually did end up registering as a childminder as I found I really enjoyed having extra children around so now I have another neighbour's children for a couple of days as well). We agreed mates' rates so I don't feel as though I'm exploiting her, but it's ever so nice to have a bit of extra pocket money - I often spend some of it on croissants for breakfast which all the children love!
So - up to you, obviously, but I just wanted to put a more positive spin on the issue. I would think seriously about the getting resentful/feeling exploited issue - but as people say, some sort of reciprocal arrangement, or agreed payment, could easily take care of that. It's nice to help people out!