This is an age old problem. The more experienced manufacturers will use a double chain guide to ensure the chain cannot skip off at the front. The decorative chain guards fitted to most may protect little fingers but will not stop the chain coming off.....

What would we do? 

Ideally the chainring would be replaced with a double guided one. As it should have been however that will require removing the pretty looking guard. 
When you have gears at the front you will always have a front mechanism that guides the chain straight on to the ring. (It's very unlikely you will be able to fit one, using adjusting screws to limit it and not using a shifter, maybe in a 24"). 
The main cause of the problem is the extreme ends of the rear gears where in gears 1 and 2, 5 and 6 the chain is forced to "cross cog" , go at a twisted angle. 

My advice is to use the limit screws on the rear mechanism to reduce the extreme gears. You may have to limit the bike to gears 2,3,4 for example.

Screwing in the L reduces the throw of the big cogs
Screwing in the H reduces the throw of the small cogs

Experiment  - But It is VERY important not to unscrew the H and L too far as this will result in the rear mechanism going into the wheel. 


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Spike Bikes