Shalom Rabbi Tahan, is a woman allowed to double knot her tichel, belt or shabbat shoes for a child on Shabbat to hold better? And does it have to be taken apart before Shabbat is over, because sometimes it stays until motzei Shabbat?

Sefaradim allow if its meant to be taken apart the same day, Ashkenazim are machmir. You can do double knot that isn’t tight, and only if you’ll open it the same day. Motsaei Shabbat for this purpose is still considered the same day.