Kittens don't groom themselves much

Oi, it's like my two sweet balls of ... smelly... fur loves don't know how to groom themselves. They're well over 4 months old so shouldn't they know how to clean themselves so they don't smell like poop in less than a week?
Tiger, the boy, doesn't mind bathes too much. Probably because of his breed that we guessed he was, Norwegian Forest or Maine Coon. But Angel, my little girl, is the complete opposite! She's a domestic short hair/oriental and she HATES water with a passion. She's so vocal and so loud in the tub that our neighbors sometimes wonder if we're torturing her. We're not, I promise! She's my baby girl but she seems to need bathes the most!

How often do you give them baths?

Are these foster kittens? I have noticed that foster kittens can be pretty gross. They don't have a mom to teach them how to groom themselves. One of my mom's foster kittens reeked until he was about a year old. He seriously freaked us out because we thought he might have been flopping on his back in the litterbox and peeing all over himself. He was often *wet* and stinky. It was pretty gross. He's mostly lucky he tricked my mom into loving him as a baby, since that was pretty nasty, even for me.

Aside from the freakshow that is my mom's foster kitten-now-grown-up-cat, a lot of foster kittens don't benefit from the teachings of a MamaCat. They take a little longer to figure out that cats are supposed to be clean. Most cats won't be gross forever, but foster kittens have a serious learning curve (even the most attentive foster-human-mom can't keep up like a legitimate MamaCat).