I always did a 'square knot' on mine with yarn.
Never had any come undone on me.
Right over left....Left over right!
Pull Tight.

My machine even has a quilting/tie stitch but it's so slow that I decided to stitch in the ditch rather than use it. It takes a stitch forward then backward to "tie". Then you have to pull up the thread and cut it after every "tie" so you can bury it later. I'd rather crawl around on the floor with my doll needle and perle cotton to tie. Even hand quilting would be faster than machine tying!

I saw Kaye Wood do that to a crib quilt. She stitched little bows in each spot. It was for a girl naturally. Really cute.

Downy says there is a risk of the ties working loose and the child swallowing them.

me too - used varigated thread and it was great.