The ovf is still going to give you the best af, compared between even the latest live view and ovf, all these other issues such as afma fade a little.
The 19 cross point system is going to be a revelation coming from the 400d that it seems a shame not to get the best out of it...
I would also recommend changing the way you use af.
I got some decent shots with my 400d by working to it's strengths, so that is selecting the centre point only, using ai servo and burst mode, tracking my subject, track, track, short burst, track, track short burst.
I used the centre point because it was the only cross point on the 400, on either the 760 or 70 you have 19 cross points to chose from, which opens up composition options.
Choice of lens is pivotal too. Very fast lenses combined with more telephoto lengths are going to give you a double whammy of depth of field issues.
The range of stm lenses are fairly limited just now, and whilst these are best for live view, usm lenses will work too, and will often be faster in ovf mode.
For a kids party I would go 50mm f1.4 or 35mm f2.0, the wider being innately more forgiving in terms of depth of field. I also find that a prime lens just lets you get on with shooting, it's one less parameter, you zoom with your feet and so get a nicer mix of perspectives, less fiddling, more shooting.
And I wouldn't use either wide open, maybe at f2.8. Shoot RAW, shoot at up to 1600 with confidence. Push the luma NR slider a little at the raw conversion stage.
The other thing you haven't mentioned, which I think for this kind of scenario is... wireless TTL flash, or even just a bounced flash.
Adding a flash will let you use faster shutters, smaller apertures, lower isos, or a combination of each. Direct flash is intrusive, bounced flash more natural. Wireless bounced flash maybe more ergonomically friendly, something like a used 420ex or earlier 430ex will be cheap, slavable, and can sit on a sideboard on its wee stand flashing up at the ceiling as you work the room.
Thanks Tinky, appreciate your many comments. They'll shure help me choosing the right gear AND help getting more good pics
I often struggled with af on the 400d, though I used the center af point about exclusively. But that may have been the case because of using slow zooms (f 3.5 - 5.6) and/or with poor lighting. So I hope to improve keeper rate on the 70d.
I'm aware of dof-issues with fast lenses. But personally I prefer a pic with a little to shallow dof than a distracting background. Maybe I change my mind when I have the experience with these lenses.
The flash thing: You're right, I forgot to mention, but don't know if flash makes sense in our party location (former tennis hall, now used as indoor playground paradise, height about 5 - 15 m), I guess no flash would work at these distances, but maybe I'll give it a try.