We have the difference in only child and age difference in our home too. But our reality is opposite of yours. My son, 14, has stepped right into the older brother role for little man, 10, and little girl,7. Just the other night little girl had a nosebleed... it was an impressive one too. Son was running around, finding washcloths, cleaning her feet where blood had splattered, offering to clean her bathroom, and showing deep concern. Son watches the little ones over the summer break. Son shares his "stuff" with them and gets their help in acting out the movies he writes. That is not to say that Son doesn't utilize alone time.... he knows how to close his door to them and say "no" to them when they knock. While neither home has massive amounts of luxuries, son has more of a home with us and more of a man cave at his father's.
In our home, we do not play "food police" where the kids have to ask for snacks and such, but they also know that if they didn't finish the previous meal they do not have the choice of snacks. The kids also know we buy walmart sprite, and they are fine with that. One prefers sweet tea over soda and the others go about 50/50 with lemonade and milk. The thing is, it is difficult for a child who is used to being the only child to be thrown into a household with other children. The child isn'tbeing greedy or thoughless when he goes and gets a snack without asking, this is how he was taught from early childhood. He may take material things for granted, but there again, this is how he was taught by his experiences. An only child is more likely to get more "stuff" because it is easier to buy for one and with one, there is the higher likelyhood that the child will be out and about with the parent.
Children are notorious for not appreciating what their parents do for them. And to a point they shouldn't have to think... wow... mom/dad really went over and above to get me a new pair of shoes.... the child should be able to depend on having shoes that fit. Children should also be able to take for granted a parent's love.... to have them question that love would mean that something has happened that has shaken them to their foundation.