You may think what you like.

Perhaps the children had found out the universal language which everyone can understand, and which wise men so far have not found.

You will have noticed long ago that they were singularly lucky children, and they may have had this piece of luck as well as others.

Or it may have been that...

but why pursue the question further?
The fact remains that in all their adventures the muddle-headed inventions which we call foreign languages never bothered them in the least.