I don't remember when was the last time that I have to go on IRS and/or my local taxation website to file payments personally. 

I'm not sure if there is such a deal structure or even the risks involved if engineered,  but the idea would be that the title doesn't really transfer / and a final balloon payment isn't made until after the property actually transfers to the children.   

I would even obtain a COFI arm permanent loan where the payments were based on an interest rate lower than what the market rates were.

A strategy I try and use is to push closing dates or pull them in to the beginning of the month to give you that extra time up front before the 1st payment is due.

.- Eyeing areas like Worcester, Shrewsbury, Grafton, and Millbury for a single-family house, priced around $500K, with a planned $100K down payment.

Next step you are selling off the annual option payment.

In this case, however, you have option two: lower the rent or have one or two renters who will help with your payment until it's fully rented.

I'm looking for suggestions, possibly even an equity partner in the deal, and/or a down payment help funding source.

If you're looking to pull all equity out it may not work, but could be an option to explore with a balloon payment and short term. 

You decide to purchase and you start making payments, alongside with other buyers.