Just to be safe until someone who knows comes along I would suggest not feeding it to the little babies. I hope someone answer soon, I hear you're supposed to get snow up there tonight :-(
horseweed has some diuretic properties and is also a potential skin irritant. It is said to take up significant amounts of cadmium when grown on soil contaminated with heavy metals and is not palatable to pigs, sheep or cows, so it is best kept away from your tortoise enclosure.