It happened around 6:30 p.m. Wednesday on the Route 611 bypass, near Broad Street.
Authorities say a Ford Mustang struck a Honda CRV in the northbound side of the bypass.
The impact of the collision caused one of the cars to roll over and another to crash into a median.
Officials say the child was a passenger in the CRV with her 55-year-old grandmother.
The grandmother was airlifted to Temple University Hospital with serious injuries.
Two other victims - the 32-year-old driver of the Mustang and his female passenger - were rushed to Abington Hospital in serious condition.
Investigators were back on the scene today looking thorough the wreckage.
The crash was so horrendous, it immediately became a criminal investigation.
David Heckler, Bucks County District Attorney said, "We will be doing a very thorough reconstruction..we are talking to some people who were on the road at the time, and we'll put it together to determine whether there was criminality."
Investigators say the Honda was in the left lane doing about 55 miles and hour when it was rear ended by the Mustang. The Honda ended up off the side of the road about 200 yards from the point of impact. The Mustang travelled another 100 yards further and landed in the median.
Investigators say the Mustang Cobra was built for speed and was fitted out with after market performance enhancers.
A witness told police he was doing about 75 miles per hour when the Mustang blew past him moments before the crash.
"Speed would appear to have been a significant factor in the causing of the collision," added Heckler."
Police are not releasing the names of the 10 year old victim and her grandmother who remains in critical condition, saying only that they are from Plumstead Township.
They're also withholding the name of the 32 year old driver of the Mustang, who lives in Pipersville.
Investigators say it may be a week before they can rule on possible criminal charges.