I don't know how careful your 7 year old son is, but if he is anything like mine was at that age, CARE is not a word that is well understood!
So, what are the implications of the likely use of a camera given your son's age.
Optical Zooms are mechanical by nature and any careless knock or drop is likely to render the zoom unserviceable.
Digital Zooms by comparison, are software animals that have no moving parts, and whilst generally inferior to optical zooms; would it be a better option for a 7 year old?
Only you can answer that question, but it's worth a thought.
As regards your budget, you can get very good quality cameras now, for less than £100. I'm sure my children would be delighted at something in that price range, even though they are now considerably older than your son, so don't be put off by that. Most of the major manufacturers have something to offer at that price point, but unless your son has exceptional skills for his age, just pick something you fancy the look of. They are all pretty similar, but I would stay with a known camera manufacturer.
If you have a local photographic store like Jessups, they are always a good source of information and often staffed by enthusiasts. At the end of the day, if you think you can get a better deal elsewhere, you can always walk away.