Yes, they actually had 4 children. Agnes, the first one, was probably stillborn, or died the same day. She was born, died and was buried in Milwaukee in July 1896.
There is a Lars Larsen Breum in the 1865 census at Jostedal, but is too young to be her father, although it is possible he may be her brother, since both of their parents may have the same names (it's impossible to be sure since the names on William and Tillie's marriage certificate are "American style"). It could be a coincidence though, because those names are all terribly, terribly common in Norway. Or he may be no relation at all. The "Breum" is the name of a place and changed if people moved (at that time).