Be open by LAW the US and Canada they can not discriminate against you. If you feel they are.. you can get them in a lot of trouble. BY LAW they have to accommodate you if they hired you. It was not necessary for you to disclose the information in the interview. If you got hired and signed to join filled out all the paperwork (pay and such) you then are safe to disclose information and by law they have to accommodate you. If you are pumping they have to give you that time.
Personally be open. Helps build relationship with colleagues etc. also chances are kids get sick and you’ll need time off for that so will come up eventually and might look odd if you had never mentioned them before?