I don't know if you have something similar there, but here in the West there is a nursery rhyme for children about the destinies or fortunes of all those born on a certain day of the week. Like, the child of Monday is supposed to be beautiful, the child of Tuesday is graceful, the child of Wednesday is woeful (which is the day Bowie was actually born on), etc. Thursday's children are supposedly the travellers, ones who have "far to go". I think Bowie pointed out on the
VH1 Storytellers live show that he actually got the title
from Ertha Kitt but I always think about the line in the Velvet Underground's "All Tomorrow's Parties" (when Nico sings "Thursday's Child is Sunday's Clown" it feels like she is saying "watch out, pride goes before a fall").
So, it's partially a reference to childhood, partially an astrological "doomed by fate" message, partially a nostalgic callback to the sixties.