pro rata in law generally means according to a calculation or apportionment formula - in the context of the FARs that formula is generally assumed to be $\left(\frac{1}{nOccupants}\right)$ as Hugh noted.
I actually went on a dive through FAA counsel opinions on how they deal with the pro rata bit and found nothing applicable to this situation, so what follows is an interpretation crafted out of my own slightly deranged mind, but it seems sensible to me.
Interestingly while digging around I found nothing in the FARs or interpretations that would seem to preclude a more complex formula for pro rata share than "divide by the number of people in the plane", but neither is there anything I can find to support doing so, and I imagine it would be much harder to justify to your local FSDO if questions came up.
For the sake of simplicity (and our collective sanity) let's stick with "simple division" as the formula.
The FARs specify that the pilot may not pay less than their pro rata share of operating expenses associated with the flight. So, if we have 4 occupants of the aircraft (Mom, Dad, Baby, and Pilot) and they go for a hundred-dollar hamburger the simple division method says pilot cannot pay less than one-fourth of the operating expenses associated with the flight. If we say the expenses work out to be \$100 (for the sake of easy math) the pilot has to pay at least \$25.
The FAA doesn't much care where the rest of the money comes from because it's implicit in the regulations that the pilot isn't keeping it (the money would be going into the hands of the FBO for fuel, oil, ramp fees, etc.) -- Mom & Dad could pay $37.50 each (covering themselves plus the baby) or split it up between themselves in any other way they wish and this is still acceptable under the regulations as long as the pilot has paid their pro rata share of the expenses.
The pilot can certainly pay more than the simple-divison pro rata share of expenses (and generally that's the most equitable option - figure out who's paying, divide by that number, and make sure the pilot is in the "paying" group to stay on the right side of the regulations).
In your example you would split the cost up among the three adults (each paying $33.33, and the pilot generally kicking in the extra penny).
The same logic in both of these arguments would apply if we weren't talking about a baby, but rather a third friend that you all decided to treat to a trip.
