The question isn't whether the child can stay in the suite (I presume that the suite is rated for 3), but whether there will be an additional charge for the child. Simply adding the child online does nothing to fix your underlying issue.

You face two problems if you do not contact the property to make arrangements and obtain an express commitment.

First, the property may simply levy a reasonable or exorbitant fee for the child. You are standing there at check-in and have no negotiating power short of walking off with your family and luggage.

Second, if the suite is needed and you are downgraded, the property reasonably believes that you are a party of two. When you show up with three, you may simply not be permitted to check in.

All of this easily solved with Skype and a polite conversation. "Will it be a problem for our 7 YOA to accompany us? I take it that there will not be any additional charge, is that correct? Would you please add her name to the reservation."