Just the plant for near where children play - not! Narrowness of leaves interesting, our out here have them much broader relative to their length. A creeping perennial, you will have to dig out the rootstocks to be rid of it.
Looks like Stinging Nettle to me too. If you brush against it, it will "sting" leaving an itchy welt. It usually goes away without lotions or doctor visits but I'd take the plant out so you don't have to worry about it.
