@123ZYX
If, for example, it's the 8 year old who had the win and you split it, what happens if the 14 year old gets a big win when she's 19? At that point, she will have gained from the 8 year old's win and won't be under any obligation to share her own. At what point does the sharing stop? Will they still have to share wins when they're all adults?
No. Because they're adults.
If I put £5k into premium bonds for each of 3DC, and one wins big, as children, I make the decision I see best for the children as I'm the adult. Otherwise children would be in charge of their bonds from the start.
As an adult, if one DC wins big, then it's down to them, to make their own adult decision.
All PB held at child ages, I would view as all in one big pot of "children's". Whichever ticket wins, is split amongst all the children. As the eldest turns 16 (I think it's 16 for PB, not 18) their share is removed from the child pot and operates independently. So if they win, it's now theirs to keep. Or make the decision to share.
If one of the two younger ones won, and the children are all close in age, I'd still give a share to the (just turned) adult child. Presuming the older child hadn't won big themselves already, and kept it. But that would only be for a very short overlap period until the second child hit 16. Once the older two were both holding independent adult bonds, I'd treat them all as separate pots. So then if the youngest won, it would be theirs. Likewise for the elder two.
Child bonds I see as for all the children, and so I would distribute fairly amongst all, as that's how I parent them in every other aspect. Adult bonds I see as for the adult child to decide.