Fertility rates in poor countries correlate heavily against the number of children dying young (http://www.bit.ly/QrJnFj), as well as against the number of malnourished children (http://www.bit.ly/QrJArU). One intuitive explanation is that if you know that only a fraction of your children will survive to adulthood, you're likely to have more of them. Of course that's not the full explanation, and another reason is that the availability of birth control and sexual education in these countries tend to be poor.
Nevertheless, I think your worried about a scenario that for the most isn't very likely. Typical aid is not just used to buy food, but also to fund education and infrastructure investments, all of which has worked to lower the fertility rates in the past.
There are other reasons to hesitate in regards to the effectiveness of aid money, but IMHO this isn't one of them. :)