I haven't heard the MA GR60, so I can't help there. I do own a pair of ATC SCM-11s, so maybe I can help some. The ATCs are great speakers, but I share Lindisfarne's reservation about the power the Onkyo can provide. It's a heavy receiver though, so I'm assuming it's got some power supply.
The ATCs are a little tricky to drive, but are generally amp friendly. As Lindisfarne mentioned, they like current AND watts. For example, my MF A5 amp, at 200 watts, can kick them around with ease, but my Naim Nait 5i, which is pretty good on current, but not on watts, barely moves the woofer. They sound thin and treble heavy with the Naim, punchy and clear with the A5.
I've powered these with a Harmon Kardon 525 some time back. That receiver had a big power supply and around 90 watts per channel in stereo and ran them rather well, but I wasn't "critical listening" either.
A big consideration is the little ones. Your girlfriend has a toddler. The SCM-11s will be on stands I reckon? They weigh about 20 pounds each with sharp corners. If the little one can topple them, it could be dangerous.
If you love the GR60s, and most of your listening will be while moving around, there is no need to worry about soundstage.
I'd keep the GR60s. It's safer with the kids, you already like them and your listening habits will be non-stationary anyway. Besides, most of what will be playing on them will be Barney's greatest hits :)