One day

One day or another I will be sitting in my wooden chair and thinking about the good old days <em>(sounds better)</em> while looking after my grandchildren...)))

one day

one day or another I will be sitting on my wood chair and thinking about my funny days, looking after my grandchilds...)))

Someday I would be sitting on my wooden chair, thinking about the old days, while looking after my grandchildren...

Someday is the better adverb to use in this case, because it denotes an indefinite time in the future. I changed the phrasing as well, to unify the actions that you will be making. In essence, seeing your grandchildren have fun made you reminisce about the fun you yourself had when you were young.

One Day

One day in the future (or another) I can imagine (will be) sitting in (on) my old wooden chair and thinking back to my youth (about my funny days,) as I look (looking) after my grandchild/ grandchildren. (grandchilds...)))

one day

one day or another I will be sitting on my woodden chair and thinking about the good old days while looking after my grandchildren...)))

one day

one day or another, I will be sitting on my wooden chair and thinking about my funny days, looking after my grandchildren...)))