Yes, they were caught on a security camera behind the Big And Tall shop that stands across the road from Elizabeth's house. While LE hasn't said that they've checked to make sure the clock on the camera is accurate, I tend to think that LE did that sort of fundamental checking and corrected for time if necessary.
If the camer was accurate or if LE compensated for a known inaccuracy, the time they were caught on tape was 12:15 pm.
If TG, the witness on the bicycle, is correct in his estimate of time, then it's impossible for the girls to have been there at that time.
However... do we know that TG's cell phone has the correct time? Someone posted here that her cell phone and her husband's cell phone differed by about 10 minutes.
Do we know that the bikes TG saw lying in the path actually belonged to Elizabeth and Lyric? If that is known, LE hasn't let on.
Are we absolutely sure of TG's estimate of the time it took for him to ride to the convenience store (about 1 minute, according to Ollipop, a verified local) and phone his daughter? According to the one source I consulted, men average 30 seconds in using a public restroom:
http://www.publicrestrooms.com/
That leaves 5 minutes unexplained (and I'm not sure I really want an explanation, considering the topic).
As a totally unscientific benchmark, I asked my husband how fast he could run 1.5 miles when he was in track in high school. This may be more relevant if I mention that he is 5'0". He said that his personal best at that distance was 7 minutes 02 seconds but he could reliably do it in 7 minutes 30 seconds.
I figure that 8 and 10 year olds could probably beat a short high school boy on foot if they were on bicycles and properly motivated, either by racing or the excitement of sneakily putting one over on Grandma.
In short, there may or may not be enough wiggle room in the conditions specified to make it possible that TG saw Elizabeth's and Lyric's bicycles.
Without more information, it's difficult to say one way or the other.