FHN I'm making the assumption that you won't and don't want any other "answer" than the one you lept to at the time and have clung to ever since. I'm also assuming anything anyone says about it will just piss you right off... So here goes nothing.
Here's some things to think about though...
Some details are missing to make logical explanations:
The length of the curtains in relation to the floor and the window sill?
Your description indicated open curtains, fully open, partially open?
Curtain material? Lined? Thin or thick?
Type and placement of the central heating? I.e an underfloor vent in the room, if so where?
Layout of the room in relation to the window, what was to the left and right of the window?
Who else was in the house that morning? Maybe someone you were unaware of?
Ages of the other 4 in the room with you?
You all stood in a line and watched the curtains, then you checked for draughts around the windows and heating vents etc and then resumed your positions in line again to watch the curtians some more?
...., then you incanted the Lady of the Tracks and the flag moved, "Oh man, did we really see what we think we just saw?" it took 2 more moves of the flag on your command to scare you all shitless to run out of the room?
Why dont you know if you were 10 or 11? You know when your birthday is and that it was January, so why can't you work out your age exactly for this? This suggests theres a period of time where you gave it little or no thought following the event and that is why it isnt burnt in exactly how old you were.
I can remember being a baby so young that I could only think in feelings. I didn't know words yet. I felt God then. He's always been with me, long before I could understand the concept of God or be told about God. He's always been as real to me as the air I breathe and the ground beneath my feet.
No, you didnt, that is projection and wishful thinking. This points to an over active imagination and poor memory filtering as well as a high bias to the supernatural, which is why you went from seeing something odd to a supernatural explanation of it in a heatbeat.
One obvious explanantion involves you being the victims of a prank much like the one cofty did to an annoying room mate, Adults or older siblings would have known about your seances and had time and motive to do this, you have now had 40 years of dreams and retelling since then to alter this into something more than it really was at the time. Yes you saw the curtains move, yes the flag appeared to move on your command, yes the candle smoked at coincidental times, the rest is projection and wishful thinking and the embellishments of retelling and having dreams about it over the years.
Another explanation is that you missed the real cause while looking for the draught that was moving the curtains. You were children, your search for the draught would have been cursory and quick as well as the fact you were not very tall. How much effort did you really put in to find the draught? Draughts can be subtle, they dont even have to be coming from the window where the cutains were. Was the room door open, shut, ajar? The other parts are coincidences, projection and your confirmation bias.
You get the picture, well tbh you probably dont as your hackles are up and you stopping reading thinking and listening as soon as your memory of god was challenged.
Sits back awaiting the vitriol....