I cannot recommend making “a deal” with your mother that would dictate to her what she may or may not do in order to take your children on a trip. You may disagree with your mother’s choices and even give her information to demonstrate the Catholic position on the issue of interfaith communion, but it is deeply problematic to manipulate her into doing as you wish by promising her an outing with her grandchildren if she complies.
My advice would be to suggest to your mother that it is not necessary to attend this Episcopal church’s services in order to tour the building. The church’s office staff would probably be pleased to arrange a tour of their church for newcomers to the area who are interested in the building for its architectural loveliness. For example, here in San Diego, many non-Catholics tour the San Diego Mission De Alcala every year because of its historical importance but do not have to attend a Catholic Mass in order to do so. I also once toured the non-Catholic National Cathedral in Washington, D.C., and did not have to attend one of its services in order to do so. If this Episcopal church near your parents is a local landmark, its staff is probably used to tourist interest in the property.
As for the issue of why Catholics do not receive Protestant communion, it is because Protestant communion is not validly confected into the body and blood of Jesus Christ because Protestant ministers do not have valid holy orders. It is also a courtesy to Protestants not to presume to receive their communion when the Catholic Church does not ordinarily allow Protestants to receive Catholic Communion. For more information on Catholics attending Protestant services, please see the links below.
Could Catholics attend Protestant services before Vatican II?
Directory for the Application of Principles and Norms on Ecumenism