I want to take teenagers to Europe and I'm pretty much stuck with summer vacation. Is earlier in the summer better...or is late June as bad as August?

this year??

If you are talking about Italy and its mega tourist hotspots then late June is IMO infinitely much better than August - not only a tad cooler weather (avg hi in Florence in August i believe is around 90 or so) but the suffocating crowds in Venice and Florence and Rome in August are much much less IME.

late June is before the big rush starting in mid to late July when most Europeans start their vacations and esp in August, the most heavily traveled month for tourists in these cities.

I'm not planning on going next summer. We're just starting the planning process right now. Anyone ever stayed on the Lido while in Venice? Is it convenient for day tripping into Venice?

I've been to the Lido and though never stayed there i would

First of all it's quite a long time-wise commute so you may not feel like going back at night, etc. which is a pity since Venice becomes even more surrealistic after dark

and the Lido has little of the romance of Venice either - just another modern beach resort.

I'd stay in Venice proper

Do try and stay IN venice.
It is so magical early in the morning.
Get the kids up early and wander. The experience is priceless.