Have you ever taken a hallucinogen? Hallucinogens don't create 'pink elephants' (by and large)... what they do is remove the filters your brain uses. So you don't see the 600 shafts of light coming off of the chandelier... you see a diffuse glow and 'sparkles'. But children (and those on hallucinogens, or certain brain injuries) see 600 shafts of light. When you move your hand, you don't see a "trail" of hands, you see a hand in movement. Children, however, usually see the trail or "tracer".
Our brains prioritize information (sight, hearing, touch, taste, smell). We actually have THOUSANDS of bits of information coming in at all times.... but most people are only aware of (to pick a general number) 10. ADHD folks are aware of about 100 (it's the primary reason why we're so distracted). Infants and small children several hundred.
The vast majority of early childhood games teach the brain HOW to filter the information it's being given (I won't go into those right now, though, or I'd go on for ages)... and they're cross cultural. Children play in the same ways all over the globe throughout history. Their brains are figuring out how to function. Distance, edges, movement, etc.
Brain scans of small children clearly show that they're "tripping" as far as sensory input goes. Their brains, as they age slowly (and by leaps and bounds) learn to prioritize the info. It starts off quite young (you can watch a baby's and then a child's 'circle of awareness' grow as they age. In part, that's because they can see more clearly. WE see the wall they're about to run headfirst into just FINE... but to them, it quite literally jumps out of nowhere.
On average children stop "tripping" most of the time by age 5. But you'll still get the "Mom, what's that black thing?"... silence... "You mean the MAILBOX?" (that you've seen 100 times) "Oh yeah. Okay. I see it now." off and on until shortly before puberty. Children will see something familiar and not recognize it. Their brain may or may not suggest something similar (or brains also search for patterns... in the absence of a pattern it will create one... hence looking for shapes in clouds).
Like others have said... I bet she's seeing either the headstones or the shadows in movement from being in the jogger, and her mind is suggesting polarbears. Grey or not, in movement things "lighten" (facet of cones and rods), and they blur to become larger.