The Beginning of my search for Albert & Lydia

When I first started researching Albert & Lydia Elsner, I knew very little.  What I knew from my Great Grandmother Lulu (Robideau) Elsner was:

1. Albert, Lydia, and Fred were born in Germany
2. Lydia's maiden name was Richter
3. They immigrated to the United States through New York
4. They lived in Chicago, Illinois before moving to Minnesota
5. They had a daughter named Elizabeth
6. They had 3 sons: Fred, Otto, and Erick
7. Erick (my ggrandpa) was born in Chicago
 
Added to that was some information that my Uncle had found when he did some research many years ago:
1. Albert Elsner's date of birth - Oct 1857, Germany
2. Lydia Richter's date of birth - Aug 1855, Germany
3. Fredrick Elsner's date of birth - Apr 1885, Germany
4. Elizabeth Elsner's date of birth - Apr 1887, Illinois
5. Otto Elsner's date of birth - Oct 1894, Illinois
6. Erick John Elsner's date of birth - 13 Oct 1896, Huron, Cook, Illinois
7. Erick's birth certificate was a belated certificate filed by Fred W. on Aug 2 or 5 in 1941.
8. Albert & Lydia married about 1884 in Germany
9. Erick & Lulu married 15 Nov 1917 in Princeton, Mille Lacs, Minnesota

I put all this information together and started my research. First I looked at the 1900 Census and found them living in Chicago, Ill.  It lists their names, relationship, race, sex, birth month and year, age, marital status, and number of years married. Albert & Lydia have been married 16 years. The next two numbers - 6/4 indicate that Lydia has had six children, four of whom are living.  This is something new!  I didn't know that Lydia had 6 children.

The next three columns indicate where each person was born, their father's birth place, then their mother's birth place. The next column is the year they immigrated - 1887. The last column is the occupation. I am always curious about the occupations of my ancestors and I see that Albert was a Saloon Keeper!  How exciting!  I wish I could have asked my grandpa about that.  He was too young to remember anything, but maybe his family talked about it now and then and he could have had some stories from those times.

So the 1900 Census has a lot of good information, but also brought up a new question for me.  What happened to the other two children?  I see that there is an age gap between Elizabeth and Otto.  Could one or both of the children have been born between 1887 and 1894?