That lies, the statement ‘the children have the turtles’ is quite right!
the child would be barnet. Also, barn is ett-word, so in plural definite form (the children) all ett-words (that ends with a consonant), ends with suffix -en. I'm a learner too, so I hope it is correct and I hope this will help you :)
So how long does it take to be able to use all these plurals fluently in everyday speech for non-native speakers?
Impossible to give a general answer. With practice, time and patience, one will learn eventually.