You do not say what is the route/plane. If you do not wish to pay in advance and have no status, then I would personally check the two adults in using OLCI and choosing them either window and aisle towards the back (if rows of 3) or the two aisles in a block of 4 towards the back. Middle seats towards the back are the least likely to be chosen by other passengers, meaning that you would be most likely to be able to sit 3 + 1 or all four together by the time you arrive at the airport. There is no miracle solution otherwise so while you can try and arrive at the departure airport as early as possible (which you could do anyway) without paying, there is simply no way to guarantee seats together for sure.