I can?t find her or her family in 1860 in Washington co or Jefferson co, IL. The largest group of Pace families were in Jefferson Co.

I did find this family in Henderson Co, IL (west of Peoria on the Iowa line)
Pace, Francis A. - 49 Va
. . . . Jane - 41 Ky
. . . . William C - 21 IL
. . . . John B - 18 IL
. . . . Mary E - 17 IL
. . . . Nancy V - 14 IL
. . . . Amanda E - 8 IL
. . . . Phebe E - 6 IL

So perhaps they were not in Illinois yet in 1860. There was a family in Tennesse that could be hers:
1860 Cheatham Co, Tn, 5th dist., PO Thomasville, p. 206
Pace, J.W. - 44 Tn
. . . . Mary - 49 Tn
. . . . F.A. (f) - 20 Tn
. . . . S.H. (f) - 19 Tn
. . . . M.A. (f) - 17 Tn
. . . . Nancy (f) - 16 Tn
. . . . Jas K (m) - 15 Tn
. . . . C.H. (m) - 12 Tn
. . . . M.C. (f) - 9 Tn (could be Mandy Caroline)
. . . . B.D. (m) - 4 Tn

She married James Willard when she was 14 and had 2 children by the time she was 18.

Source for birth/death dates: tombstone, Jolliff Cemetery.

Find A Grave Memorial# 15547566
http://www.findagrave.com/cgi-bin/fg.cgi?page=gr&GRid=15547566

• Married: 14 JUL 1865 in Washington Co, IL

1. Octavia C. WILLARD b: 6 APR 1867 in Illinois
2. Martin WILLARD b: 1868 in maybe in Illinois
3. John George WILLARD b: MAY 1872 in Mo
4. James Perry WILLARD b: MAY 1875 in Ark
5. Caroline L. "Etta" Or ?Call? WILLARD b: NOV 1877 in Oregon Co, MO
6. Posey Woodside WILLARD b: 24 SEP 1879 in Oregon Co, MO
7. Charley S. WILLARD b: 14 MAR 1885 in Oregon Co, MO
8. Franklin Manworn "Frank" WILLARD b: 14 MAR 1885 in Oregon Co, MO
9. Randol Casey WILLARD b: NOV 1886 in Oregon Co, MO
10. Elijah Harrison ?Lige? WILLARD b: 8 OCT 1887 in Oregon Co, MO