I spend a lot of time correcting my kids while they are speaking. Not just when they use the wrong word, but also when they mispronounce words. I have this theory that we write the way we speak, and we tend to spell words the way we say them. So if one constantly says blatter, instead of bladder they will always spell it incorrectly with out ever realizing they are doing it.
I'm not rude when I correct them. I don't interrupt them mid-sentence. Usually I repeat back what they just said, inserting the correction when I do. When I don't feel like being a parrot, I just correct the one word they used/said incorrectly. Like this morning Nico said "The dog just swinged me around.", and my reply was simply "Swung". Then he repeated "Swung" and we went on with our morning. No big deal, my kids are used to it, and take it in stride.
The nice part is I don't have to spend hours sitting them down and teaching them all the rules of past tense, and which words are exceptions to the rules. If they get corrected enough times, they will incorporate the exceptions on their own.
It's more difficult for me to correct Lucas on his pronunciation because of his early speech delays. I always assume he's trying to say it correctly, but is having trouble forming the correct sounds. After last night, I am reevaluating that assumption, and I will make more of an effort to get him to pronounce things correctly. I can't remember what we were talking about, but Lucas said the word crotch, only he said it more like quatch. Timmy and I were laughing, and commenting on how cute his little "accent" is, and trying to mimic the way Lucas had said it. After a bit Lucas jumps in and says "It's quatch with a 'q'." That's when it hit me, his mispronunciation isn't because he has difficulty saying the 'cr' combination, but because he actually thinks or hears 'cr' as 'q'. I immediately explained to him that it's actually crotch, with a 'cr' not a 'q'.
Although mispronunciations can be incredibly adorable, you are not doing your kids any favors by letting them continue.
(Side note, I always encourage my kids to speak in complete sentences using proper English, not slang. Again; I believe you write the way you speak, and if you speak using proper English, you will naturally write that way as well.)