Cover bulky airplane seat belt buckles with a tube sock
The last time I flew with my 2.5 year old, she was obsessed with her huge belt buckle. On a recent cross-country flight, we cut a tube sock and covered it up so that she wouldn't be tempted to open her belt. Worked like a charm! It also added a little extra padding-- a "crazy leg" or leg warmer could work too.
Related: Most of what's in the Travel/Going Out archive