If your original DVD don't be still scratched for your childrens, only must to rip audio, don't need to reencode whole video. So, if your actual audio is corrupted in some way for splits, encodings and reencodings, and like as you say reencoding produces noise, maybe you can solve this problem ripping audio again.

dialhot,
Done!!!
I did it BeSweet way. As I'm not used to use BeSweet it took a little time to me. But I don't mind, maybe I'm using it more times

Ofended? I didn't want to.

And prodater, no, BeSweet doesn't re-encode, it just transcodes: in 15-20 seconds I had each new audio track ready.

Then I remuxed with BBMpeg. After reading dialhot note, I tried with TMPGEnc, but in this case I got bigger files with TMPGEnc than with BBMpeg.

And finally joined back with TMPGEnc. And: perfect.

Thank you everybody for the help: dialhot, kwag, prodater. I you all.

Cheers

Not offended, what do you say?
Thank you for besweet hint. I don't knew that. Maybe you can tell me how to make it?

prodater,
I'm using BeSweet GUI version 0.7... or something.
I'll PM you when I have time, because I'll explain myself better in spanish

OK. Gracias.