The ads at the top of the page are from Google - it is called Google Adsense and web owners (ie, people just like Mini and Pierce) put Adsense onto their site and when people click on the ads, the owners (ie, Mini) will get a few pence. The few pences all add up to contributing towards the running costs of a site like this - the web hosting for forums is quite expensive and this is one way of keeping membership free but having a contribution towards the running costs. Note this is running costs not salary - the site is manned 100% by volunteers.
Adsense (or Adwords) work by picking up relevant "keywords" in text. So if I were to talk in the post a lot about babies, possibly six month olds and under and about the different types of prams and cots available then the chances are that the key words at the top would pick up on the words babies, cots, prams, etc, and relevant ads would appear at the top.
Unless the MiniMins site owners were to put "exception" rules into this, then lots of diet related adwords would come up at the top in rotational order or sometimes just in random order.