The heartbroken mother and grandmother of Connor Horan have pleaded with members of the public to shed light on the death of the two-year-old.

Connor was being babysat in August 2018, when he was taken to hospital with multiple head and internal injuries.

He was declared dead at the hospital on August 19, 2018.

Police said they were told he had fallen down a small set of stairs.

Child Trauma Unit detectives launched an investigation into his death after a post mortem examination, and are still trying to piece together the timeline of events from the weekend Connor died.

Two-year-old Connor Horan died in 2018. ( Supplied: Queensland Police Service )

Connor's mother Emily Horan recounted her last memory of the toddler in an appeal for information from the public.

"I said 'who's mummy?' and he pointed to his heart — that is one memory that I'll never forget," she said.

"There was never a day [that] goes by that he would not cheer you up, or he wouldn't do anything to just at least put a smile on your face.

"He was always full of joy and kindness."

Connor's grandmother Deborah Ballard said he was so beautiful and happy.

"The last time I saw my grandson … he was so happy to see me," Ms Ballard said.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 37 seconds 37 s Emily Horan recounts her last memory of her two-year-old son Connor.

"He has his little happy dance that he does, and gosh, we miss that so much.

"He came up to me three times that day doing his little happy dance … putting his arms up in the air to get a cuddle from grandma."

She said the family just wants to know what happened to him.

"He was loved by so many, and we miss him so much. It's destroyed our family, not knowing what's happened. Please, please come forward," she said.

Detective Acting Inspector Matt Howard said the circumstances surrounding the case were being treated as highly suspicious.

"We've been treating it that way for some time now," he said.

Anyone with information is being asked by police to contact Crime Stoppers.