SEA - I think there are a fair few people who see younger children as easier, and needing to know less 'stuff', therefore easier to teach.
Have a think about your own experience of children at home though. Would you have said the easiest stage was the newborn baby phase, because they are younger? Have you entertained two toddlers at home, and then two eight year olds? I would have the eight year olds any day - they know basically what is required of them in terms of behaviour, and can get on with stuff themselves.
I have experience of teaching all ages from nursery up to sixth form (not in a school but in a place where school visits took place). I love little kids. But they are much harder work than the big kids. Big kids already know the 'rules' (hands up for speaking so we can all hear each other for example),little kids don't. So no, you are not delivering in depth info, but you are constantly interrupting the flow of the lesson with comments like:
Don't sit on his lap - well, because he doesn't seem to like it. Just sit on your own bottom on the floor.
Sit down, please. No, just sit down. Because we're all looking at this and we need to be sitting down to see it properly.
Don't worry, we can clean it up, but next time maybe try and tell someone that you need the toilet, ok?
And: Put your hand up... hold on, I haven't asked the question yet. Ok, I'm pleased so many of you can put your hands up. Put them down for a moment. Now, I'm going to ask a question and if you think you know the answer, don't say anything, don't tell me, keep it a secret but show me you know by putting your hand in the air. Ok?
Year 4s are much easier to manage. You don't need to 'parent' them in quite the same way (eg you don't need to ask them every few minutes if they need to go to the loo because you will be away from a toilet for 10 minutes - they understand concepts like 'go now because you won't have a chance for a few minutes!'). Largely, they can get their own clothing off and on (coats, trousers with stubborn buttons which cause accidents). They recognise their own belongings - amazing how many tiny children swear blind a coat isn't theirs and because they can't read the name label carefully sewed in won't have it that yes, it does in fact belong to them...
But mainly, in this early stage, they are being taught to read. The most fundamental skill in education. Get that wrong and what secondary school they go to will be the least of your problems. Getting a very young person to understand that the funny shapes on the page correspond to sounds, which build up into words, especially when a lot of those shapes sound different depending on what other shapes are around them, especially when said young child is more worried about when he's going to eat lunch, or whether someone else will play with the bricks he was playing with when you called him away to do this 'reading' thing - that's hard hard going.