Am I overreacting?

My bf & I agreed no cereal in our five week old baby’s bottle even though his mom was really pushing for it with the reasoning that she did it with all her babies & it makes them sleep better

Last night I fell asleep while he had the baby and I woke up around midnight surprised that the baby hadn’t woke up to eat again cause normally he wakes every two to three hours four hours being the longest stretch he’s ever slept between feeds so I go to make a bottle and when I’m cleaning his bottles I see one half drunken and full of cereal I was so pissed I told my bf and his mom that they would never watch him while I wasn’t around again because I can’t trust them and they care more about their sleep than his well being I kind of feel like I overreacted but I’m still very upset because my baby already had tummy troubles from being premature and me having to switch him from breast milk to formula only a week ago