Since you are using babies born as the basis for the ranking you should be able to have it do a check of babies per hour, if it is unreasonably high the stat doesn't get accepted, that does of course making an educated guess as to what is reasonable.

Such as my game has been going for 959 real hours with 836 babies born, thats just .87 per hour average. While it would be possible to get higher average it can't be much higher unless there are a lot of disasters wiping out the bulk of the population, after all you can't have more babies after you reach a population of 90 until someone dies so that and the fact you have to improve farming before you do too much with the poopulation or everyone starves keeps a limit on the number of possible babies per hour ratio. You would just have to develope and keep secret what the actual number is to reduce the cheaters.

Also a check of real hours played would get the system clock cheaters off the board though that would be more involved to keep it advancing so as not to block legitimate players.