I think "le" can be considered as meaning of "change", their child maybe already turned to five or six years old. "ba" shows the uncertainty.

"le" in the first sentence can be considered as a sign of the present perfect tense, while "Ta men jia de hai zi wu liu sui" simply means the kid is 5 or 6 years old right now. As for "ba", it means the inference of the speaker. But if he ends the sentence with "ma", it is asking with no certainty.

Tāmen jiā de háizi wǔ liù suì le ba. the speaker is guessing how old the child is Tāmen de háizi wǔ liù suì ?? the speaker is asking how old the child is

about.. “ta men jia ”. Of their children, not on behalf of their children. “ta men jia de hai zi”PErhaps his brother's children or sister's child

The "ba" on behalf of the uncertainty