Join Date: Aug 2013
Location: Cincinnati, OH
Bikes: 2013 Trek 520, 2014 Trek Stache 8, 1985 Nishiki Team Issue, 2014 Dahon Vigor P9, State Vice 2.0
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24" is the largest of what will typically be considered a true "kids' bike," whereas the 26" wheeled bikes you mention will probably be adult bikes. The latter will usually (though not always) have better quality frames and components, and ride more efficiently than a kid's bike. If you were to bring your girls into my shop, we'd all recommend the adult bike, and most likely the 13" frame size (maybe the 15" depending on their growth rate and bike-handling skills). If they're already at 4'11", they would quickly outgrow a 24" bike, and you'd be right back in the same situation before too long. The larger bikes, however, will offer more room to grow, giving the bikes greater longevity for your girls.
When you're in the adult bike category, wheel size is more an indicator of how the bike will ride/handle, and/or for what sort of terrain it's best suited, while the frame size is the more important determining factor for general fit. There are some who might disagree, but with the right frame size and geometry, wheel size shouldn't really matter at all for fit. Road bikes and more pavement-friendly hybrids will typically have 29"/700c wheels, while 26" wheels will usually be reserved for more quick-handling, technical-trail kinds of mountain bikes, or for some cruisers and such. Given the kind of riding you're doing, I might recommend something along the lines of a mountain-esque hybrid, like the Trek Dual Sport/Neko
family of bikes.
In general, you'll want to get your daughters onto a couple of different bikes to try them on for size. Let them compare the 24" to the adult-size bikes, and see if you can take some for a spin. I'm willing to bet they'll be amazed at the difference from their little kids' bikes. In any case, best of luck to you in finding the right rides for your twins!