The safe rule to draw from these considerations, I think, is that one should never direct even the slightest word of criticism toward a mother who brings her young children to Mass.
As one of those Moms, I appreciate the consideration. Frankly, I'm awed when the same people choose to sit near us day after day. It took me years to stop being grumpy with my own children at Mass (Ok, ds as the older got the brunt, but if you know him you also understand how that could happen!). The many words of encouragement I've heard are very validating.