The obvious reason that Jesus told His disciples not to hinder the children from coming to Him was and is that they understand His message better than any adult can. As soon as a child is old enough to know that He can’t earn salvation, they know that they have to ask Jesus into their heart and their life. It doesn’t take a lot of sermons or education for a child to just “get it”, they do.
And they get it far better than we do. After we become adults, if we haven’t truly accepted Jesus as our Savior, we are so covered in education and pride and all sorts of other “filters” until we can barely see the message of the cross, let alone the real message of salvation. The world itself places those filters on us and our hearts and we allow it to happen. Most of the time we don’t even realize that it is happening, just as when we were sinners.
We didn’t know that we needed Jesus at that time and we didn’t really feel the need for Him. Until the Spirit of God touched our hearts and we noticed all of the sins we had committed and that they were separating us from God, most of us didn’t even know that we needed Him. We were comfortable with our lives and the sin that we lived in.
Don’t think that your life and your righteousness is good enough because it isn’t! If Moses couldn’t get into the Promised land because of his sin, then we are not close to being able to get into Heaven without Jesus! Regardless of which religion you follow or have been raised in, your goodness, your righteousness is as filthy rags next to the grace, mercy and forgiveness which comes from your belief in Jesus!