Your sentence is: Back from Kids’ Time 2021 in Kielce, Poland.
This phrase is correct as a short statement, but it is a sentence fragment because it doesn’t have a subject or a verb. If you want to make it a complete sentence, you can say something like:
- I am back from Kids’ Time 2021 in Kielce, Poland.
- We just came back from Kids’ Time 2021 in Kielce, Poland.
These sentences include a subject and a verb, which makes them complete sentences.
If you want to keep the phrase as a title or a note, it’s perfectly fine to use it as is!