As we all know all too well, Elly's maturity was constantly questioned during the first few years of the strip's run. Not only did Jim and Marian remind her how childish she acted most of the time, John treated her like an infant that needed protecting. Hell, even she herself occasionally despaired that she would never really mature. This fear of being an infant forever was bad enough in and of itself; the negative impact it had on her own children made it worse. It seems to me that a case might be made that she more-or-less unconsciously fled from the company of her children in order to not be lumped in with them by the rest of the world. We know that a stranger that happened to encounter her and the kids would see a slightly disheveled and oddly unobservant mother and her kids; her fear is that said observer would see three idiot children.