Depends on which kids we're talking about. Pfizer will likely be approved for use in 12-15 year olds in Canada and the US inside the next month or so. The approval application was submitted to Health Canada at least a couple of weeks ago.
In Canada, we'll have enough vaccine supply by the end of June to cover the entire (currently) eligible population with a single dose, and two-thirds with both. Assuming supply doesn't slow down significantly into July, I 100% expect vaccinations to open up to 12+ year olds in the summer. Probably pretty early in the summer. And there's no reason to assume a slowdown in supply in July - they're expecting something like 120 million total doses by the end of September, which means July-September alone will see more doses delivered (almost 1.5x) than the total delivered by the end of June. And some of those will be one-shot vaccines.
They might not all have two shots by the time school starts, but almost certainly one.
And most people will not be waiting 16 weeks for a second shot, even though that might well turn out to be better for longer term immunity. Those who got their first shot in early March, when the policy changed, might get close. Anyone getting their first shot in May is probably looking at closer to an 8-week timeline. By mid-June they'll be at or near the 4-week spacing. All assuming that provincial distribution can keep up with supply, and so far they've given us no reason to believe they can't.
It's not optimism. Just arithmetic.