If they’re not in mortal sin, they’re not in mortal sin, and that means they’re free to receive Communion. You can provide instruction about the rules for receiving Communion, you can teach them what constitutes mortal sin, you can answer whatever questions they ask you about receiving Communion. But I strongly urge you not to presume to tell them they cannot receive Communion because you “suspect” they are in mortal sin. Provide them with opportunities to go to confession on a regular basis (such as at a once per month family excursion to confession), and assist them in examining their conscience during family devotions (by offering questions for them to think about, like “Have I acted chastely in all of my relationships?”), but otherwise trust that between their confessor and God’s grace, they’ll be given all the assistance they need to know when they are in mortal sin and should not receive Communion until their next confession.
