"All mice are small."
Translation:Alle muizen zijn klein.
"Elk" is best translated as "each". It refers to every individual within a group. I think it is mostly synonymous to "ieder" (every)? Somebody correct me if I'm wrong. Some examples:
"Alle kinderen krijgen een snoepje" - "All children get a candy"
This could theoretically mean that they have to share one piece of candy! That would be terrible.
"Elk kind krijgt een snoepje" - "Each child gets a candy"
This makes it completely clear that every individual child gets his/her own candy.
Beloeng: It's simple: The English said 'All'. Or did you read 'Every?? Apart from that, I doubt whether 'Iedere' can go with a plural noun. Just like 'Every' will only work with 'mouse', so 'Iedere' would only go with 'muiz'.