That would change my intuition quite a bit. Even assuming 100 kids were feasible in a human lifespan, the odds of 99 girls or 100 girls happening is (1 [arrangement where all the kids are girls] + 100 [arrangements where there are 99 girls and 1 boy]) / 2^100, or about 8e-29. Practically zero.

If we assume that each child really did have a 50/50 chance of being boy or girl, then the result would be that there's a 1/101 chance that it's 100 girls.

Given what I know about the world and genetics and such, I think it's much more likely that there's some predisposition by the couple to have girls.

If we think it's, say, 90/10, then the prior probability of the 100-girls case would be 0.9 ^ 100, and the prior probability of each of the 99-girls cases would be 0.9^990.1 -- i.e. the all-girls case is 9x more likely. 0.9^100 / (100 0.9^990.1 + 0.9^100) = 0.9 / (1000.1 + 0.9) = 0.9/10.9 = about 8% probability of having 100 girls, 92% probability of having 99 girls and 1 boy.

If we think the couple has 99:1 odds of girl:boy on each birth, then it's 0.99^100 / (100 * 0.99^990.01 + 0.99^100) or about 50/50 on whether they have 99 girls or 100 girls.

If we think the odds are 999:1, then it's 0.999^100 / (100 0.999^99*0.001 + 0.999^100) = around 90% chance they have 100 girls.

Someone else can do the math assuming an uninformative prior on the couple's girl:boy odds and calculating the posterior distribution given that we know there are 99 girls.