I've never thought of this before, but I wonder of Jesus was nervous about how the disciples' mission would go. Maybe it would all end in disaster - they'd try and fail to heal someone and then blame it on their "lack of faith." Or they'd commit some awful heresy in their preaching: "God loves everybody, except you."
So when they all came back with big grins on their faces, maybe there was a massive sense of relief for him.
If I read his prayer in that light, perhaps it gives me a clue as to what he was thinking while they were away. "Oh Father, what have I done? I've entrusted my mission to people who have as much clue as babies! They haven't begun to understand what we're doing, Father. Is it all going to go wrong?"
But back come these babies, and Jesus finds that God trusted them with his truth, primed them with his power, and asserted his authority through them. Jesus as he had through Jesus. The prophets had longed to see God at work in this way, and never had. And this bunch of fishermen and tax collectors, with not a GSCE in RE between them have seen into the heart of God's plan for the world and faithfully proclaimed it to everyone they met.
Wow! Thank you God!
In the end, it's all about trust. Jesus trusts his Father, the disciples trust Jesus, and God the heavenly Father trusts the disciples.