Bronson Battersby's neighbour recalled seeing the toddler "waving" out the window to her just days before he was found dead alongside his father.
Cheree Ross, who may have been the last person to see two-year-old Bronson alive, said the boy had a "mouthful of pink wafer" as he waved on Boxing Day when she checked in on them.
The toddler was found dead two weeks later next to the body of his 60-year-old father, Kenneth Battersby, who had suffered a fatal heart attack at the flat they shared in Skegness.
It is believed that Mr Battersby died days after Boxing Day, leaving Bronson with no access to food or water at their home on Prince Alfred Avenue.
"It got to Boxing Day and we thought, hold on - where's Kenny," she told Sky News, with her family often seeing the pair together on the street.
Mr Battersby was "sorting bits in the kitchen" while they said hello to Bronson for what would turn out to be the final time.
Ms Ross said she stood outside the house for two hours when paramedics were called to the house on 9 January, after a social worker was led into the property by the landlord.
"It was devastating. I saw paramedics turn up with the police... never did I think that it was Bronson," she said, adding she was "made aware" of Mr Battersby.
"Like an idiot, I stood here for two hours with a pair of gloves that Bronson liked to wear, still covered in chocolate, just waiting for them to bring him out the house."
Ms Ross said the worst was confirmed to her when the child's mother, Sarah Piesse, who did not live with the pair, "shouted" Bronson was dead.
"I fell to the ground in tears, it shouldn't have happened to the boy," she said.
"The whole street is in shock, and we're all wondering why we didn't do more."
Investigation into police response
The police watchdog will investigate whether there were any "missed opportunities" by officers prior to their deaths.
Lincolnshire Police was contacted on two separate occasions by a Lincolnshire county council social worker who had no answer when they knocked on Mr Battersby's door.
Bronson was known to children's services and would typically be seen at least once a month by social workers.
A spokesman for the county council confirmed a social worker communicated with Mr Battersby on 27 December and arranged a visit for 2 January, but there was no response at the door.
The social worker "made inquiries at other addresses where the child could be" and contacted the police, before a second unannounced visit on 4 January.
The days since the deaths have been "horrific", Ms Ross said, but she hoped to push for a law - Bronson's Law - to introduce a weekly point of call for children below school age living with an elderly or vulnerable parent.
"I feel as though... you can save lives and we can prevent another tragedy," she said.
"I would ask Keir Starmer [Labour Party leader] to please consider this."
'A beautiful relationship'
Ms Ross described the father and son as "amazing" and said "you'd never see a dad love a son so much and vice-versa".
"They were always together, completely inseparable and it was a laugh a minute with the two of them," she said.
"I remember Bronson standing on the washing machine door and breaking it, and the commotion of Kenny trying to get it out - he ended up breaking his foot, he was hobbling about for weeks."
"My children haven't had an active father in their life and neither did I, so to see a dad just dote on his child, it was amazing," she added.
"For an older gentleman as well, they loved each other. It was a beautiful relationship."
A minute's silence will be held for the pair ahead of their beloved Torquay United's match with Dover Athletic on Saturday.