I think as a general rule, a confident kitten isn't likely to become unconfident unless something happens to make them that way, although confidence doesn't always equate to cuddliness. On the other hand, a shy kitten can certainly be brought out of itself in the right environment and often forms a very strong bond with their slave.
In your circumstances with children in the house, I'd certainly recommend a bold, confident kitten that has been well socialised (preferrably with children) by the breeder. There are no guarantees but you will get a good idea at 12 weeks by observing the kitten and asking questions about the environment it's been raised in.
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