I'm thinking of buying my son a small fish bowl, but I am a complete novice and would welcome any advice on what fish to buy, what accessories I require and how to maintain it.
Good starter fish for kids are betta fish and goldfish that being said fish bowls are not adequate for fish, as much as its advertised. Unless it's big enough to install a filter and heater while still having plenty of room for your fish to live.
5gal is a good start to house 1 betta fish, if you want to have a goldfish you'll need to upgrade as they can become pretty big.
Bettas need a filter and a heater where as goldfish only need a filter. You'll need water conditioner, small water syphon (tiny pump to suck up feces in the gravel), algae scrubber and food.
Your new tank should be filtered and conditioned/cycled for minimum 1 week before introducing fish, this allows time for beneficial bacteria to grow.
When doing water changes, only change 20% of water (water changes are achieved when syphoning feces). This is done approx. every 2 weeks.
Live plants are very good for fish but need care as well by adding UV lighting and plant food. Fake plants are safe and your best choice for a childs tank.
Algae will grown in the tank very fast if it's exposed to sunlight, green algae is not harmful but good to clean if it's getting out of control.