In my understanding, 達{たち} is a suffix used to pluralize a countable noun. For example,
According to most dictionaries, 子供 can be translated as either child or children. As a result, if we want to say children (instead of child), I think 子供 should be enough. In other words, is it true that for saying "children", たち in 子供たち is redundant?
No. I suppose it's a bit like you guys. It serves to make it clear there are multiple children.
Japanese language doesn't have plural form of noun like English. So we can't know how many children are playing in the park in this sentence "子供が公園で遊んでいます".
If you want to say "A child is playing in the park", you say "一人の子供が公園で遊んでいます。"
If you want to say "Children are playing in the park", you say "子供たちが公園で遊んでいます。".
It is explicit, to state that there are several children.
