Get in there with them. Tell me your doubt. I would tell them over and over again, "It's not the strength of your faith that saves you. It's the strength of the one who your faith is in. It's not your faith that saves you. It's Him that saves you." I would talk to them, "Yeah, tell me about your doubt. Let me hear about it." Then I wouldn't get angry.
I think as parents when we hear our kids have doubts we get angry and fearful. That's not the answer. The answer is to come beside them and say, "Okay baby, I get it. If I'm being honest, I have doubts too, so let's talk about your doubts. Let's go to the word, let's go to God's specific word to you speaking to your doubts and then let's pray because ultimately He's gonna have to be the one that gives you the faith, and the Bible says that faith's a gift. He's gonna have to be the one to give you the faith to believe."
I would encourage your child not to always be wondering how much do I believe? Is it a lot, is it a little? Again, going back to it's not about how much or how little you believe, it's about who you believe in. We're all gonna go through times of doubt. I would share that with your kids. There was a time that I doubted the exact same way you do, and guess what? He loved me through it. How? How was he faithful when I'm faithless? That's what makes Him so beautiful.
Again, not shaming them. Encouraging them, bringing them back to the good news of forgiveness, restorations, God's love that's not dependent on their performance over, and over, and over again.