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 :)
That lies, the statement ‘the children have the turtles’ is quite right!
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.