My kids got this as a Christmas present two years ago. I even suggested this as a gift to my mother-in-law since I was sure the kids would love to create forts and trains and such (we got the train set too). Their interest waned quickly (7, 5, and 2 years old at the time). Although the concept is great, the product is less than perfect and just ridiculously too hard for the kids. To build anything more than one square high is asking for trouble. The foam inside is too squishy and soft... the structures always collapse. Like another reviewer mentioned, it needs to be more rigid or thicker. The velcro is INCREDIBLY tough. Unfortunately, it's way too tough. It binds quickly to the fabric, but that also means that when you tear the velcro off from another piece, it quickly binds to its own square... then you have to tear it off again. Sounds simple enough, right? I've torn up my own knuckles trying to open these velcro pieces. After pulling open two or three strips of velcro, my fingers are sore and I want to give up. My kids didn't have the strength to do it themselves. I normally don't complain much about toys in these reviews, but I was surprised to see so many good reviews on this toy. Perhaps our set was made differently than some of the others'? My kids are now 9, 7, 4 and 3 and I finally dragged these unused toys from a box in the rec room where they have been stored untouched for 2 years now. I'm donating them.
So, in summary... they are not dangerous, but are just not made well enough to justify the cost.