I just bought this house and am trying to make sure everything in my yard is safe for my kids. Can you tell me what this plant is? It is about 3 feet tall, and seems to have burr-like clusters near the back. I'm in Southern California, if that helps. No idea whether it was planted intentionally. 
Here are more images. http://imgur.com/a/xzgjO
I'm pretty sure it's Ricinus communis; its not entirely clear from the photo because of the way the light falls, but as you've got young children, I suggest you remove it. The seeds are highly toxic, but the biggest problem with this plant is its fine pollen when in flower, which can cause asthma attacks. Allergic responses to the leaves and stems are not uncommon either, so although its generally safe if you don't eat it, I wouldn't choose to grow it where there are young children. As you don't know whether it was deliberately planted or not, its probably wise to keep a check next year in case it's seeded itself and others pop up, though if you live somewhere with colder winters, that's much less likely. More info here https://en.m.wikipedia.org/wiki/Ricinus
Also know as Castor Bean. It is toxic to pets as well. I would dig it out as deeply as possible and then monitor the area for offshoots. Dig those out as soon as they come up.
That is a castor bean tree. The leaves are called the hand of God because it is thought they have healing properties. The red spikey balls on them contain seeds, which contain a poison called ricin. They are very dangerous; if ingested, ricin will kill you. The seeds look like kidney beans. I save a few seeds for next year. It is a dangerous plant if you have kids that try to eat them.
