Let's not forget that the kids would be missed.
They would stop showing up at school and stop being seen around the neighborhood where they lived. Also, kids coming into but not leaving a foreign country could cause problems for their guardian. Whoever brought them to Europe would be the first person the police would go to.
The guardians probably don't want to get involved with child smuggling.
We know from the script that the kids live with Miles' ex-partner Marie, and she's not a fan of Miles and Cobb.
COBB: ... I thought you could talk to
Marie about bringing them on
vacation. Somewhere I could meet
MILES : Why would she listen to me?
COBB : You were married for twenty years.
MILES : She blames me as much as you.
We also know that Cobb's activities have landed him in legal trouble and that Marie is aware of that.
COBB Doesn't she understand that my kids
MILES : Yes, she does. We all do. Go back
and face the music.
COBB : Be realistic, Stephen. They'd never
understand− they'd lock me up and
throwaway the key. Or worse.
MILES : You think what you're doing now is
helping your case?
COBB : Lawyers don't pay for themselves.
This is what I have. This is what
you taught me.
It's seems unlikely that after divorcing Miles for his involvement, she would be too keen on an international child-smuggling operation.
Even if Cobb managed to arrange some sort of legal cover-up using his connections, he'd still have to contend with Marie (the guardian). If she was a good caretaker of the children, she likely wouldn't stop looking for the kids if they vanished or were taken from her. This would require Cobb to kill or incapacitate his (ex) mother-in-law, which would be out of character for him.