'attend' doesn't take a preposition, so only number 2 is correct. I would change number 1 to... 'The child was failing to attend school regularly.'

Here is how I would phrase these. 1. The child was failing to regularly attend school. Or, The child was failing to attend school regularly. These two sentences seem very formal. I would just say, the child wasn't attending school regularly. 2. Perfect!!! 3. Our children may attend the same school. (May shows potential). It's possible that our kids may attend the same school (It's possible shows potential).