My oldest boy got his scout when he was 5. He's now 11 and it still fits him. A cricket or chipmunk are just way to small for him to shoot properly now and no he's not ready for an adult one yet.
By 11, our kids were already shooting their FR8's with no troubles. Their Chipmunks were pretty much safe bound at that point, and had been for a number of years.
It's heavier? yes but do you really need your 5 year old walking around with it?
Our kids at 4 were shooting offhand, on their own, with theirs, so yea, I did want a gun they could properly handle. They also were shooting from sitting and prone field positions at the same age. This is something that no other gun at the time, and most likely even now, will/would allow. To learn properly, you need a gun that fits, all the other "youth" models at the time, just would not allow that. Even today, I really havent seen anything else that does.
We used the Chipmunks as trainers, which they are well suited for, if you want to teach your kids to shoot properly from the git go (I never let them shoot off a bench as kids, other than sitting or lying on top of one, they only ever shot from field positions, prone, sitting, and offhand). I knew they would be growing out of them, and that really wasnt an issue. What they gained from their experience with them, was worth well more than the little the rifles cost.
When they are around 4 or 5 I always think about getting one but at that age they are more suited to bb guns they just don't have the proper attention span for it. By 6 they can fit a scout and my kids are not on the large size.
We never allowed our kids to have BB guns. My dad wouldnt allow us to have them either, and his reasoning was right, we treated them as toys. Had some great wars too!
Worked the same with our kids. They had "real" guns, right off, and knew what they were, even at a very young age. We never had any problems in that respect.
Attention span is up to you to work with. Work within it, and take full advantage of it, and youll have no troubles. I worked with our kids pretty much every day from the time they could hold their rfles, and they dry fired with me using their rifles, each night (I was shooting HP pretty regular then, and was constantly dry firing in the house). When they fired their first "live" rounds at 4, they had "fired" thousands of rounds in the living room in practice. Their first live rounds were fired offhand from a chair in an indoor range at 10 yards. Most all of them were in the black of a standard pistol target too.