Would something like this work?
https://www.amazon.com/Childproof-Toilet-Handle-2-Pack-Button/dp/B071X52DPZ/ref=mp_s_a_1_3?crid=1917073RTE06W&keywords=flushstop&qid=1679072675&sprefix=flushstop%2Caps%2C94&sr=8-3