I'm not sure I understand what you are trying to do. While I don't use the FindChildByXpath method myself, as I understand it, it's purpose is not to alter data in a table but to find an object on a page using the XPath. So, based upon the code you posted, I'm not sure how you're actually doing what you are trying to do.
Just as a guess... if you want to find a bunch of objects using XPath and increment specific portions of your xpath based upon a for loop, I'd write it like this:
for (var i=0; i < maxValue; i++){
myObject = page.FindChildByXpath("//div[1]/div[2]/div[2]/div/div/div/div[2]/div[1]/div/table/tbody/tr/td/table/tbody/tr[5]/td/div/div[3]/table/tbody[1]/tr[ " + i + "]/td[1]/div/span/input",true);
//execute whatever you need to against myObject
}