which is another approach. how about for each child you get automatically accounted a number of years of payments as if you had earned money. if it takes 30 years of work to earn enough for a decent pension, and every child counts for 5 years then 3 children would allow you to get the same pension with only 15 years of work. how is that?
btw, the global infertility rate is 5%. the global disability rate is 10%. those people are also affected by not being able to work. in other words, 15% of the population are unable to fully contribute to pension funds. where a full contribution means work and having children. but making children a pension contribution actually helps a number of disabled who are still able to have children. so now they are able to contribute more than they would if we ignored children as a contribution.
seriously, this constant shooting down of ideas just because it appears that one group is disadvantaged is tiring. try thinking a bit more creatively and help come up with a solution.