I have two kids, and it's fairly easy (at least before starting school / sports)
1 - Second hands clothes, toys, etc are super cheap 1.1 - Hand me down across the extended family and your family (seriously, they grow out of it in minutes, try not to buy it new) 2 - Your kids don't need a lot of toys and crayons and paper are cheap 3 - GO TO THE LIBRARY 4 - Parks are really fun, and really cheap, as in free
Where it adds up, at least for me, is I now have a 2 & 4 year old in school and that's $1K a month (drops to zero once they hit public schools), but for a few years that's hefty. However, that's also a choice
1 - I chose private school for preschool, so that's costlier 2 - I moved to a specific area for the 10/10 public schools to save money