I can offer some details on Xxxholic, as I'm more familiar with that one than others, though I've also read all of Cardcaptor Sakura and the majority of Tsubasa REservoir Chronicle.
Spoilers ahead if you haven't read CCS, TRC or Xxxholic
Clow, who is deceased at the time of TRC and Xxxholic, is an ancestor of Sakura's (with no indication of how far back that is). Clow is sort of reincarnated in two characters in CCS, one of whom is Eriol (little kid with glasses who looks like a chibi Clow). The "original" Sakura and Syaoran, from CCS, grew up and married and apparently felt their son (I haven't seen any conclusive name, but he's a boy) was in danger, so they split him into "clones" (could be a bad translation) and sent each into different universes. Two that I know of are named Syaoran, and both show up in TRC. Watanuki from Xxxholic is a third. I don't know of any more, but I haven't read the entire CLAMPiverse and they're still writing. I can't draw a family tree here but if it were tipped on its side:
Sakura = Syaoran
Watanuki "Other Syaorans"
It's highly unlikely Clow and Yuuko had a child together. I get the impression she's quite old (her clothing, pipe and furnishings suggest late 19th century at the latest) but she seems not nearly as old and certainly not as powerful as Clow. Yuuko and Clow made Mokona (both of them) foreseeing they'd be needed for communication between Yuuko and the TSC main characters.
I hope this helps a bit.