Going back to the Zoe app. I use it for me, but I don't report my children, because they would be useless at reporting. This would be every day in my house:
DD1: I'm fine don't fuss
DD2: Yes! I have everything. Tick every box for me... I feel terrible!
Ds: I'm fine.
So I don't think they'd be much help!
But also I think at one point it said cases "between 16 and 65" so would they count them?
The thing about teenagers being hospitalised may just be an effect of so many having Covid. If 0.1% need hospitalisation, then if 1000 teenagers are ill, then 1 is in hospital. If 50 000 teenagers are ill then there's 50, which stops looking quite so "no worries". Add to that the number of teenagers with health concerns: Remember that health concerns include diabetes, asthma, obesity and other conditions that many teens have. And the fact that children who should be shielding were told to go back to school. 
My ds had his appendix out in June. I'm told by the consultant he is at risk still if he gets it. Not a major risk, but it was enough to give him the flu jab and I've been told if he gets it, first sign of breathing issues to get him to A&E.
What proportion of teens have underlying health conditions? I don't know, but isn't diabetes and asthma both less than 1 in 10 children? Say that's 3 in 20 have one or both... out of 50k, that's 7500 children. Children's departments often aren't very big, and with covid the capacity has been brought down at our local one at least, because they're only using every other bed, it's basically halved.
I'm also not sure how they would, barring using the 6 private rooms, be able to separate the covid patients from the others in the children's department, as the wards are very open, and you walk past them to get anywhere. It may be they've taken a separate ward from elsewhere in the hospital where they can be segregated more easily. But that may also reduce capacity simply on trained paediatric staff who can't be in two places at once.
I think this new strain has really caught them on the hop. There was clearly something up by Friday's figures. I was hoping we were on the way out a week ago. A lot of change in a week.
But I'm really looking forward to pointing out to the rest of my family that they go into tier 3 and stick. We go in, and come out again within a few days. They even create a new tier for us. We are truly privileged 