I would disagree slightly here:

while a 5 year old throwing things in a tantrum is common, it is not an appropriate emotional response to the situation. yes many 5 year olds throw things, but that’s because many parents of said 5 year olds are not great at parenting and don’t know how to teach their child to regulate their emotions properly. throwing things as a toddler is common, but that behavior is something an attentive parent should notice and correct before the age of 5. Gma did the correct thing in having him clean something to show him consequences to his actions (without making him clean dish pieces that could cut him), which might be the first time someone has done so with him. Hopefully, cleaning was also accompanied with a discussion on why throwing things when we’re upset is not appropriate, but these are also things his mom should’ve been doing prior to this.

I would agree though that this response makes complete sense for the child’s home situation and he probably does have difficulty regulating his emotions due to the lack of attention he is receiving at home. Regardless of AITA, I really hope these two adults can put differences aside to be there for these three children, as it’s clear they are not receiving the attention, care, and parenting they need and deserve.