I know that Dorothy has two children: Michael and Kate

...but how many children do Rose and Blanche have, and what are their names?

I know Rose has Kirsten (played by two different people) and Brigit, but there is also mention of Janella (in the deck of cards) and Charlie Jr.

I know Blanche has Becky (played by two different people of very different sizes) and Janet (who she speaks to on the phone), but she also mentions Skippy more than once and also makes mention of Biff and Doug.

Blanche also mentioned a son Matthew. I think it was mentioned in the episode "To Catch A Neighbor" (s2)

According to Wiki...

Rose had 5 children: three daughters (Kirsten, Bridgette, and Jeanella) and two sons (Adam and Charlie Jr.).

Blanche possibly had 6 children: two daughters (Rebecca and Janet, both of whom made appearances on the show) and four sons (including Matthew, a CPA who appeared in the spin-off series The Golden Palace).[2] Biff, Doug, Matthew, and Skippy were mentioned as her sons once to Dorothy but never mentioned again.