Seems the children would have gone to school together. I assume they continued going to their same school after they became a family

Did they live in the same part of town? What they do outside of like dance lessons, riding, swimming lessons, and maybe seeing each other at the park?
I would say that they didn't know eachother before their parents met.

Based on how they dealt with each other in the wedding and its immediate aftermath, I think it's likely the Brady boys and then-Martin girls were not only unknown to each other before Mike and Carol met, but at most only got introduced to each other when their respective parents decided to marry (and maybe got taken to a solitary joint restaurant outing before the nuptials). It seemed that the kids had gotten to know their parents' fiancees far more than they had their future opposite gender legal sibs (and barely knew more about them besides their names and birth orders before the 'I dos').

They said in the pilot that Carols parents lived “20 minutes away”. And given that the Brady kids walked to school, they likely had neighborhood schools and were in a different school district than the one the Tylers lived in. However, they later said Carol went to Westdale, so I imagine that the Tylers lived in the same city - just perhaps at opposite ends. If Carol and Mike hadn’t gotten married, maybe the kids would have run into each other in high school.