Is my child at higher risk for giardiasis at day care?

Somewhat. At a given time one or more infants can have minimal symptoms of giardia & be in a day care center. Poor technique may lead to contamination of a variety of surfaces including a changing table & hands tainted with the microscopic cysts may bring them to the mouth where the cyst enters the body to begin the infestation.
Not likely. Giardiasis is a water born parasite. I would not expect the water supply of a day care center to be contaminated.