Not the flowers but what made them choose Yvonne De Carlo over the Marshal woman?
Or even the role of Eddie? I wonder what was the deciding factor in picking Butch Patrick and not the other boy.

They picked Yvonne De Carlo over Joan Marshall because they felt that Joan Marshall looked too much like Carolyn Jones on The Addams Family.

They picked Butch Patrick over Happy Derman, because the producers felt that Happy Derman was too mean and grouchy of a child, and got rid of him for a much nicer Butch Patrick.

I have a feeling that Happy Derman's Eddie was more a reflection of the Eddie character in the original treaments or scripts by Chris Hayward and Al Burns or Ed Haas and Norm Liebmann. I guess I don't really know, but I can't imagine Happy Derman intuitively turning the character into a freakish wolf child without any guidance.

All I've ever seen of JM and HD was in the FG bio.
HD did appear to act wolf-like but all someone had to do was coach the kid! :D

Munsters#1,
I would think that it was that Joan Marshall looked liked the Morticia character than looking like Carolyn Jones because the Addams Family TV series was in production at the same as the Munsters and hadn't been aired yet.
The Addams Family had a good runner-up in Joan Marshall if Carolyn Jones had been unavailiable.
Butch Patrick was a likable kid. Stephen Cox, in his book, compared Happy Derman's portrayal of Eddie to Linda Blair in the Exorcist. Where they would of gone with that, I don't know. Or they couldn't sustain the character being like that, so they made Eddie harmless afterward.
Yvonne DeCarlo was alot more charismatic and expressive than Joan Marshall.