You're all reacting to the wrong part of this. The blood sample part is unimportant, they could have just dug in the garbage and gotten a dirty diaper or whatever. They sent the blood off for DNA typing, which they could have done on any other thing that kid had touched instead. Follow him to McDonalds, collect the straw from his juicebox when it gets thrown away, end result is the same, and no warrant is needed for that either.