You don't say how far you are riding or how far the 8yo is already riding. But 8 is old enough to do a lot. I wouldn't buy a trailer for him. It's certainly not going to take him 3 years to build endurance. My 5yo can ride three miles no problem on his 16er, I haven't found his limit yet. And he's... not exactly the next soccer superstar.
There may not be an off-the-shelf solution for your 6yo that will last you. Have a look at the seat Martyn Ashton had made for his 'Random Tandem" or maybe take a look at Bilenky / Hase tandems.
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Genesis 49:16-17