This reminds me of another time the Lord was traveling from Galilee to the Jordan area. Except this time he traveled through Samaria. Jesus cut right through on a straight path to a well outside of town for an important encounter. There was a woman coming to the well to draw water.
Jesus was tired. Physically tired from walking in the hot desert sun and now apparently hungry and thirsty. The disciples go off into town to buy some food as Jesus sits down by the well. It's noon and the well should be vacant, but it's not.
The point is this: even though Jesus was weary from his traveling, his teaching schedule and constant discussions with the pharisees he still did the will of the Father. He was still doing ministry.
Jesus goes on to explain to this woman all the details of her life and really takes her to the Living Well. This is in spite of his physical condition. He worked past it. At the end of the passage we catch a glimpse of how he was able to accomplish this. The disciples didn't understand. They thought someone else brought him food to eat.
If God has called you to something He will be faithful to equip you as you serve. The hard part is staying committed to the calling. Continually doing the will of the Father has some amazing benefits both here on earth and later for heaven.
So back to why the disciples were telling parents to keep the kids away from Jesus. I think the disciples were tired also and just hadn't figured out how to continually do the will of the Father and not be weary of the ministry. One bonus we have now is the power of the Holy Spirit in us to give us power to accomplish His will. There are lots of things in this world that can cause us to become tired. We just have to keep our focus so we don't loose sight of the prize ahead. In children's ministry it's no different. It seems like a revolving door. No one wants the job of Sunday school teacher. The job of teaching kids isn't the most visible position of leadership. Yet even Jesus wasn't too busy to lay hands and pray over the children. We shouldn't be either.