Panaracer Flataway liners are not heavy nor stiff.

I've seen good reviews on them. I have sent a pair up to my nephew to try out. But, I don't know how much riding he does. I had him on road tires earlier, and it was a major hassle with the goatheads.

I'm hoping that with a combination of tough tires (Michelin Protek Cross Max), and the tire lines, and Michelin Protek Self sealing tubes, that his flats will be a thing of the past. Of course, that makes a heavy, slow combination on the road.

I don't use tire liners here. My Protek Cross Max tires got stolen, but they always seemed a bit slow. Nonetheless, no flats.

I have a Schwalbe One on my folding bike. Sweet tire, but it seems to be quickly getting riddled with holes. So, perhaps I'll have to consider additional protection.