Consider: When I was single I lived a very very very comfortable life on $22,000 a year (cash, not income). I put that much in my checking account and everything else in savings and investment. This was after I spent several years putting the majority of my pay on loans to become debt free and paid off the $60,000 in student loans and a very small car loan.
If someone makes a salary of $30,000 a year than they probably take home around $22,000 after taxes and insurance. It would be much less comfortable for them to have a lot of savings. True, not impossible, but much much harder than me since their takehome pay is equal to my "frugal" pay. They would have to give up much more than I do. Obviously you can't max out your 401(k) and IRA on that salary, maxing those out would be more than your take home pay! Basically - not possible until you get past median income (which is around $50k)
As an aside I don't think children have to be that expensive... Granted I don't have them but I see all my friends spending a massive amount of money on very silly things for their kids. Nobody needs the majority of crap that is bought for kids. Household size has, IIR, doubled in the last 60 years. It isn't like kids' needs have changed to take up double space since my grandparents were raising them - it is parents just give them more.