Prior to a FF digital camera, all professionals were getting excellent results with the smaller sensor camera's. Are we saying now that it is not possible without a FF camera?
To the OP....it's your clients opinion that count. If you can get that "perfect" exposure, and able to "capture the moments" with the kids, that's all parents would ask for. Don't mortgage your house, and don't over think it.
Most of them were using MF film cameras

Medium format film is a larger capture area than 24x36, and as such, it gives a shallower depth of field at each and every angle of view. Please parse that statement carefully: A) shallower depth of field at B)every angle of view. Medium format film is larger than most of today's medium-format digital backs, which are much larger than "Full Frame Digital" or FX digital, which is roughly 24x36mm sensor size.
Somebody said that professionals did excellent work before FF digital: not quite. They did 'acceptable' work using crop-sensor digitals, but the majority of serious,higher-end pros went to FF digital as soon as they could, because it brings back 1) lens focal lengths to the angles of view they learned on and 2) it provides less depth of field at each ANGLE OF VIEW and so 3) FF digital gives better subject/background isolation potential on every photo op.
One of the things consumers equate with professional photos is the ability to take , "a clear shot with a soft, fuzzy background." I used to shoot in a portrait studio 5/8,and that is one of the things many higher-end consumers expect to see: foreground/background isolation. Medium format excels at that. 24x36 is pretty good at that. Indoors, APS-C sucks at that, and the work that comes out of an APS-C camera on things like groups shots, looks substandard to me, compared with work shot on MF film,MF digital, or 24x26 film or digital. if you want your work to LOOK BETTER than the cheap-o places, you need to start with the right tool: a camera designed for the lens system the maker markets, and not a temporary stop-gap size format that was all makers could manage to create until a few years ago.