I recently brought these for both my kids. My daughter is almost 3 and my son is 4 months. I paid approx £10 a pair.
After a particularly stressful beach trip which mainly consisted of my daughter complaining that her sunglases kept falling off, and having to constantly turn the baby away from the sun because his hat wasn't shading his eyes well enough, I decided enough was enough. I went to our local surf shop and explained the problem and our good friend in there recommended these to me.
The basically consist of a pair of kids sunglasses with the added elastic band which goes around their head, thus holding the galsses in place. I was slightly sceptical at first, thinking my daughter and son would just reach up and snap them off their head in 5 seconds. However, when I got home and gave my daughter her pair (pretty pink colour) she immediately asked me to put them on for her. And that is where they stayed. My four month old hasn't tried to remove his blue pair either.

They come in a variety of colours and the sizes increase by age which is great. I would definately recommend these to anyone whose kids are fed up of trying to keep their sunglasses on while running aroundon the beach.