On a budget, I like Wellgo MG-1s. I run a set on one of my own bikes, and I've bought them as gifts for my son and various of his friends over the years. eBay is the best source. They come in various colors, which is fun for the kids.

Specialized makes a set of plastic, BMX-style pedals. Your local shop might stock them. I am not so sure about the plastic though. All my own pedals are metal.

Really, any decent-quality pinned pedal will do the job. They are a great choice if you just want to be able to hop on your bike and ride. (Just don't go barefoot. LOL!)