I don't mean to sound unsymapthetic, but if you want to help her, then suggesting to her that a redesign will be very useful would be the route I would advise. Besides, I happen to believe that even 70 year olds are still able to learn new tricks.
Things like grouping on specific values, which you inquired about, are just one of the reasons to do so.
You could create a work-around with a union query that combines each of the existing "children" and "birthdate" fields into a single field, and then apply your criteria to that.
Still, it's a short-run/long-run thing. Paste a band-aid on it, or fix it properly. If she really doesn't want to go for a proper fix, you're stuck with the UNION query band-aid.