I’m assuming that since she’s MY granddaughter - she is a Catholic. I would remind her what she was raised knowing – that she was saved at Baptism when she was an infant (John 3:5, Mark 16:16, Acts 2:38, 2 Pet. 3:21).
I would also remind her that since we are flawed humans with a free will that we sometimes stumble in sin. I would tell her that the Scriptures warn us that we can fall away and lose our security if we turn away from God - so we need to cooperate with His grace, remain faithful and do His will (Matt. 7:21).
If she has Scriptural questions about that - I would gladly show her (Matt. 7:19-23, Matt. 10:22, Matt. 24:13, Matt. 25:31–46, John 15:1-6, Rom. 11:22, 1 Cor. 4:4, 1 Cor. 9:27, 1 Cor. 10:12, 1 Tim. 4:1, 1 Tim. 4:16, 2 Tim. 2:12, Heb. 3:6, Heb. 3:12-14, Heb 6:4-6, Heb. 10:26-27, 2 Pet. 2:20-21, 2 Pet. 3:17, 1 John 2:24, 1 John 5:13, Rev. 3:5, Rev. 22:19).
A good analogy would be the following:
Salvation is like a boat that comes along and saves you when you’re lost at sea. You are safe and secure – as long as you STAY in the boat – but you are free to jump off at ANY point and LOSE your security.
Hope that helps.