Staying more than stated in the UK visitor application

[Vik0916475]

Hi
My parents came here in UK on visitor visa in Aug2016 and they stated in the application that they will be here for 3 weeks but instead they stayed for 6 months (but did not overstay). This was just because they came here to see their grandson and wanted to spend more time with him, that's it, no other reason.
Now, I want to apply for their visit visa again next year but after reading the consequences for staying longer than stated, I am bit concerned because for IO (immigration officer), this is straight like "undermining the credibility" however, this was certainly was not the intention.

Next is, how did they support themselves here so it is totally simple, they had more than £10k which was more than enough but they even did not use it as they were staying with us.

Second is, what happened to India ties, employment, house and relatives, so for that it was again not complicated, my sister live 2mins walk from our house and she did take care of that and about employment, my father is a retired government employee so the place he worked had no issue in taking Unpaid leaves as it was more about my father's credibility and my father joined the job after went back to India (meantime they hired someone else).

It was quite simple to explain here but I want to know if someone else had the same situation and did apply for visa again so what happened and if it was rejected what did you do to get that approved or it is just one way, once credibility has undermined, you are done and visa will not be granted again.

Really need advise please....

Regards,
Vik