Can I feed toddler with leftover meals? (Like one-day-old rice turned chocolate porridge)

I wouldn't advise feeding leftovers to a child if you could help it. There is bacteria growth which multiplies rapidly overnight and this is not worth the risk of your baby falling ill. You can freeze and thaw out tiny portions if you have a lot of food instead of cooking everything and keeping them for the next day.

Normally I will cook a fresh porridge with fresh ingredients (vegetables/meat). I do not encourage to feed overnight leftover rice to baby as we do not want to risk the little tummy and cause upset, especially if bacteria grow due to the mishandling (heat and storage).

it will not be advisable to feed children leftovers as their tummy isn't as "tough" as ours. It might cause a tummy upset or gassy tummy. I will cook them "fresh" everyday so as to minimise them being sick.