Check out LensCrafters if you have one nearby. I worked there many years ago and if I recall correctly they have a really good replacement plan (I think it used to be $40 or so for a year and they will literally replace them every month for free if needed for just that one fee upfront.) They also tend to have a lot of variety and a pretty good selection of children's frames.
Most places will want to put your DS in polycarbonate lenses, because they are the least breakable and therefore safest for kids. As far as safety goes I totally agree, but some people have trouble seeing thru polycarb (I'm one who totally can't see thru it at all, but I have a REALLY high prescription and that makes a big difference.) If your DS has problems with fuzziness or weird colors in his field of vision in polycarb lenses, ask to switch to regular plastic (CR-39). (It's really just as safe in *most* cases.)
Look in those "money mailers" or "valpak" coupons you get in the mail for coupons for vision places. Most places have pretty good package deals on glasses and exam, especially for kids. Also try websites of the larger chains for coupons and specials.
