issues!

Any help would be appreciated. This is my very first bike project of any kind. Don't know much but learning quickly. Trying to finish this Bonanza. Connecting the chain to the rear sprocket....started engine...ran great for about a minute then it through the chain off the rear...

From last night... My 3 year old son likes to park his two next to dads. He would not get in the pic for me though. Mind you I picked my 1st one up last summer (the Yellow Arctic Cat Climber where I am still waiting on my seat from my brother-in-law for the second time) I know...