I designed these creepy little things in Tinkercad for the kids. After printing, there's a pry point at the end of the butt/tail on the smaller one to get you started in removing it from your print bed. Then fold and snap (or cut) off the long leg anchors. The centipede is around 140mm square, which is about as big as I can print on my Qidi X-One. They'll scale down a bit if you need it smaller. And they certainly scale up! They print nicely hollow with two perimeter layers, but I ended up printing them at 100% infill for more creepy weight. Printing at less than 0.200 layer height may cause adhesion problems where the feet attach (i.e. a leg may fall over before connecting with the body)
I design and print stuff for my kids. I hope yours will enjoy them too!