What word would be used for as a gender-neutral offspring, for when the gender doesn't matter or is unknown, like "Your child," or "My children"?
Дитина - child
plural of дитина is діти
You can always
how is ь in донька changing the pronunciation of н ?
I don't know how the soft sign is supposed to sound since the situations with it always seem to have it.
нь like ñ in Spanish piña, н like usual English n
What is the difference between the two forms of "daughter"?
No difference, дочка, донька - you can use any. Just mind the difference in stress: дочкА, дОнька (audio for this sentences has wrong pronunciation of донька)
If you'll say "донькА" nobody will mind :)