The Spirit will draw your children to Himself. What you need to do is be ready to tell them how much He loves them and how precious they are to Him. That He loves them even more than you do. Speak to them of His greatness, mercy and grace...His wonderful love. What you are doing is preparing the soil of their hearts to receive the incorruptible seed of Christ, which is eternal.
For little children, keep the message of salvation simple. Ask them if they would like Jesus to come into their heart and be their friend. Tell them that Jesus is your friend and He wants to be their friend. All of our children asked Jesus into their lives as very young children. We prayed for them to be filled with the Holy Spirit and they all spoke in tongues. As they matured their experience deepened. All of them wanted to be baptized when they were in elementary school. We didn't make them understand the fullness of what they were doing. The Holy Spirit was prompting them. That's what happened to us when we were baptized. I had no great revelation as to why I wanted to be baptized I just knew I wanted to be. Don't expect the theology to be completely understood for God looks at the heart.
Just like Samuel was a child when the Holy Spirit called to him expect your child to hear from the Lord. When the Lord first called Samuel in 1Samuel 3 he didn't recognize whose voice it was. After running to Eli three times in response to the call, Eli finally figured out that the Lord was calling Samuel. He informed Samuel that it was the Lord’s voice he was hearing and then instructed him to go and wait to hear it again. When he did he was to say ‘here I am Lord, I am listening’. In the same way, we need to instruct our children to listen and then ask them what the Lord said. Never devalue or criticize what your child says. Instead, encourage them that they can hear and that God wants to speak to them.
Children believe that God can do and will do whatever they ask. How simple is that? Jesus said if you ask anything in my name I will do it. Children don't have the unbelief of the world system blocking their thinking. For this reason when they are told that they can lay hands on the sick and they will recover, they do it and believe it will transpire. Children can and will do great exploits. All they need to know is that they can and He will do the rest. It's that simple - for the kingdom of God belongs to these!