If it's not in the decree, you don't have to pay anything. If she goes to have it put in the decree, it depends upon your state law.
In Missouri, daycare costs are considered a support expense, and are figured into the formula. For example: If you each make $2,500 a month and have 2 kids, the support amount would be $545. If the CP pays $600 a month in daycare, support amount would be $805, so you're paying an extra $255. If you were making $5,000 and she was still at $2,500 your support would go from $906 to $1,253, an increase of $347.
For Missouri child support calculations, see
http://www.rollanet.org/~hclaw/child_support_form-01_03.html.