yeah i ran my 20" bike without a chain tensioner for about a year.
the problem is that sometimes the chain will go loose, tight, loose as it rotates.
if you don't get the chain tension adjusted just right without a tensioner, the chain can be either too loose when it gets to the "loose" stage of it's rotation, or too tight when it gets to the "tight" stage of it's rotation.
the chain needs to be neither too loose or too tight in it's full rotation so it doesn;t come off, or break.
it can be done but it takes a few tries to get the chain adjusted to the right tightness.
you will probably also find out that you won't be able to get the pedal chain adjusted and the engine chain adjusted so that they are both right. one may be too loose, while the other may be too tight.
This can be a challenge in itself, and sometimes you have to use a 1/2 link in one of the chains to get them both adjusted right.
this is where the tensioner really helps because it takes up the extra slack in the engine chain, while allowing the pedal chain to stay tight.