The UK is already over 90% previously infected or vaxxed and still showing antibodies. We're not at HIT even within subgroups. It's very unlikely that there is a realistic HIT of simultaneous immunity that can push the R rate of Delta reliably below 1 - especially in countries where life is spent largely indoors for months at a time.
In the UK, we are approaching what looks like an endemic balance with vaccine boosters providing something close to temporary immunity and pushing the disease into a much milder form for almost everyone.
On the broader point about vaccinating kids for the good of the herd. I don't buy it, it has to remain a risk-benefit analysis for children. Though I'm happy to include things like open schools, no limitations on play/sports/socialising, no masks and similar mitigations, no limits on travel, no fear of accidentally killing granny etc in the benefit column.
The individual driven analysis always has to be done though. The UK regulators are cautious on the subject - they still haven't approved a second dose for 12-15s for example. They may never do it. Personally, I'm glad about that, it means they take it seriously.
