I rode a children's bike, now the chain is sagging - is it ok to shorten it? I know that I shouldn't have, but I had a go on my son's bike (https://www.giant-bicycles.com/au/xtc-street-20-2012).

I think that I stretched the chain. It's sagging now with about 3" of vertical play at the point between the cogs. 

So - can I buy a chain breaker and just shorten the chain, or should I get a new one?
If this happened over the course of 1 ride, it sounds like the wheel just slipped forward in the dropouts. If you loosen both nuts on the rear wheel axle and pull it backwards evenly, you should be able to retension the chain to proper working specs. Then just tighten it back to a torque of pretty f**ckin tight so it doesn’t slip again!