Since I am raising my two preschool age boys in a Spanish-speaking country, I am their main source of English. Basically they get English from me, educational shows, childrens' shows and movies. This I use to my advantage in lots of ways. For example, I hate the word fart. I think it is a gross-sounding word. My sons have always said toot since they were old enough to know what that was, and since they are boys that means since they started speaking. Once on a visit to my brother's house in the States, Alex, my older son, asked for a bedtime story that just happened to be about farts. Well, during the whole story my brother could NOT figure out why he wasn't laughing. He kept reading the book and staring at Alex like he was a little slow or had no sense of humor. That's when I leaned over and whispered to him to replace the word fart with toot. He did and Alex about fell on the floor he was laughing so hard.
I have also tried to keep the insults to a minimum by at least not giving them ammunition for name-callng. For a long time they called each other cacahuate (peanut) since I guess they couldn't find a good insult in English. They now know words like dumb and stupid although they don't often use them because they don't hear anybody using words like that. Let me rephrase that: I try not to let them hear me when I say words I wouldn't want them repeating. When Alex was a toddler I remember yelling at another driver. I called the guy stupid and then stopped short because I realized that was a word Alex had never heard before. He noticed the silence and began repeating words he thought I had said. "Toopid, coopid, soopid", he went on all the way home. He never quite got it though, and since Baby Einstein was all he watched at the time, it took him a while to learn that specific word.
Sometimes my boys make up their own words. Just recently I walked into the kitchen while they were eating breakfast and heard them saying, "I'm going to fork you!" and sword fighting with their forks. That's a new one. Also Alex has recently been saying "Holy Foxes", when something suprises him. I think he must have heard that one from something he watched.
I realize that someday their vocabulary will contain all the crass insults and words kids learn in school that parents wish they didn't. I am not into censorship or over-protecting my sons, I just figure as long as I'm the one teaching, I may as well teach things worth saying for now. Sometimes I slip up. The other day Alex was very angry about something and said, "It's just freakin'. You are freakin', Mommy." Oops.