I'm writing story the involves introducing characters in a flashback as children and am having trouble thinking of how to do this any suggestions or tips?
Why do they have to be introduced in flashback? I'd say it would be much easier to introduce them in the present and then flash back. Or do they only turn up in flashbacks? If they must be introduced in flashback, I would say start a new chapter for each character, and each chapter opens with a childhood flashback before cutting to the 'present day'.
Why the flashback? Is this truly integral to the story or just a convenient way to tell something from the past? (You want to be careful and use flashback devices cautiously.) There are any number of ways to introduce characters in flashback. I have one story (where the past history of a group of, now adult, schoolmates is the focus of a murder uncovered many years later). One character who still lives in the old hometown learns the area where the body is buried is about to be excavated. He calls his closest childhood chum (and partner in crime, so to speak) to alert him to the pending 'discovery'. This leads to the remembrance kind of flashback which leads back to the childhood events leading to the memory and the future discovery of the remains. The five chums, certain one among them will be the certain suspect in the murder, come together to figure out how to handle/conceal the crime. In yet another manuscript - my current WIP - the entire story revolves around flashbacks. It only moves forward when it is looking back, so to speak. There are a lot of things that trigger memories, from a photo in an album or the mention of a name long ago forgotten, to a particular scent or a sound or phrase that made a long ago impression. Think about what takes you back in time and go from there.
I think triggering flashbacks by some cue element is a bit cliche. The memory works in a hello-lot more complex way than that. Things brew in the subconscious for anything from seconds to years and may appear out of the blue. Let them happen naturally, where it fits the mood and pace of the story and don't worry about overly obvious cues like photo albums, or seeing a pencil on a table and then remember your first day at school.
