Thank you for your question
My name is Clare
I shall do my best to help you but I need some further information first
How old are the chidlren and where has she taken them and why?
Where has she taken them?
Which one - this is important?
Do you have any reason to think that they will not be brought back?
Is she Serbian and what is your nationality?
the biggest question is - Do you believe she will return in four weeks of is there a good chance she wont.
This will influence what the next steps are
Your ex was not entitled to take the children abroad without your agreement or an Order from the Court but it will only be classed as Child Abduction if it is clear that she has no intention of returning the children to the UK.
If they do return then you can apply for a Prohibited Steps order to prevent any repetition.
If she breaks that Order then it will be an offence.
If she does not in fact return you can apply to the courts in Serbia for their return to the UK using the Hague Convention to which Serbia is a signatory
Please ask if you need further details