Wear the nicest appropriate thing in your closet -- no t-shirts, short skirts, etc. I don't even own a suit, so I'll be in slacks and a sport coat. You should not feel like you need to buy clothes, but you also shouldn't be dressed like you're heading to the grocery store.
No gum, no food or drink (unless medically required).
No talking in the pews. Sometimes, the congregation will get bored for a longer rite and begin chatting. Don't let that be our group -- if you need to discuss something, please go to the narthex (entrance) of the church. A short yes/no question that is whispered is one thing -- chatting about your friends and some upcoming plans is another.
Please share all of these rules with your families but especially let them know about the Catholic Church's stance that non-Catholics should not receive Communion. They may either remain seated or go forward and cross themselves (arms crossed on chest with their hands on the opposite shoulder) and the minister will offer them a blessing. If this upsets them, I can try to pastorally explain why -- there are good and valid reasons even though it may come across as unfriendly and exclusionary. The overly short answer is that to receive Communion implies that a person is "in communion" with the beliefs of that church.
Depending on the church, there may be rules against photography ... or it may only be against flash photography ... but even if nothing is mentioned: discretely coming forward, kneeling in the aisle, and taking a picture or two is one thing -- holding iPads up in front of everyone to video the whole ceremony is another.