One of the things that Paul hears on occasion is that children do not/cannot understand sermons. I have an analogy that I think can be carried over into our Christian life.
Nelson is 8. He has little knowledge of how cars work. But, Paul takes Nelson to the van and shows him various parts using the jargon of a mechanic. Paul can explain the purpose and job of spark plugs. He might explain the job of the carburetor (because our van is old and has one). And, when certain acrid smells burn inside our noses, Paul will teach Nelson about the radiator. At the beginning of these lessons the jargon is liken to a foreign language. Nelson will not understand how a motor makes the van move. But, in time, these concepts will take hold... and Nelson will be making car repairs on his own.
Rebecca is 10. I have worked with her some to begin teaching the rudiments of piano. When she began taking lessons, she did not know the meaning of the words: note, scale, octave, interval, chord, etc. But, in using these words regularly (and with illustrations) Rebecca has learned some of the meanings of and names for the musical forms that she hears. In the beginning she cannot play a song. But, in time she will build upon her knowledge and she will make music.
In church situations, many people think that children cannot be expected to listen to a sermon or teaching that is not geared solely for children. The argument is that kids will not understand the big words and the difficult concepts... after all, we adults don't always understand everything! (For example - the trinity, or atonement, or the virgin birth.) But, teachers and preachers need to speak in a way that will bring the hearers to new understanding and new knowledge. Preachers take the milk and meat of the Word and they break it down into chewable portions. Ideally, a sermon will contain some milk (simple words of explanation focusing on the basic tenets of faith and simple applications) as well as some meat (challenging abstract ideas, doctrines, and more challenging applications). In this way, the child (or new believer) will grow to understand as they hear more teaching/preaching.
Will a child sitting through a church service understand everything he hears? Not likely. But, it is building a foundation and a framework to be built upon in the future.