Spikes from above! How close should they get?

This year, I want to make a 4'x8' room (passage) that traps the kids, then spikes lower from the ceiling. I have two questions. First, how strong should I make the walls? I have images of kids banging on the walls trying to escape. The second is, how close should the (flexible foam) spikes get to the kids? I want them to get close but I don't want the kids to start grabbing at them. I have a 10' ceiling to start with. My thought is to put the spikes on with velcro so that they just rip off without damage if the kids get grabby. My other thought is to lower the spikes to 6', then switch to a strobe light and start moving some visual clues (pictures on the walls) up so that it looks like the spike are lowering but they are really staying a safe distance.

Any thoughts/ideas?