Group child initialization 

This should rectify this issue https://github.com/web-animations/web-animations-next/issues/260

@shans @dstockwell ptal
> Hmm, do we still even need to push startTimes down? (logic on line 121)

We either need to do that or set currentTime. Seems like 6 of one and half a dozen of the other. I experimented with setting currentTime so that we could remove the null startTime test but it doesn't want to work properly.