Touring with kids - tag along and kids trailer behind same bike I am wondering if anyone has experience of pulling tag-along and then kids trailer? Would that be possible to tag a kids trailer to a 20 inch tag along?
Sounds like a bad idea. The biggest concern would be how the setup would act under hard braking, especially when going down an incline. There will a lot of mass to stop which means your stopping distance will be very long. Additionally the trailer and tag-along chain will be unstable under braking and may jack-knife. 

Update:

If you are going to try this in a controlled fashion as Criggie suggests in his answer, Make sure your brakes are in good shape and properly adjusted first. Having a pro mechanic at a bike shop inspect and set them up would be a good idea.