Liaisons aren't used in slow speech just as a rule of thumb in French.
Taken from the "Tips & Notes" section in the Common Phrases lesson (desktop only): "Also, when speaking slowly, liaisons are often omitted. This is why liaisons disappear in the slow versions of listening exercises. Be careful of this."
L'ours = loors Leurs = lehuhrz (z gets pronounced depending on the next word, see liaisons)
Ours-which is both singular and plural for bear/bears- always gets the 's' pronounced, no matter the next word, i.e. l'ours noir = loors newahr, l'ours a une pomme = loors ah une pomme
This isn't the case for leur, which might sound like lehuhr or lehuhrz depending on the next word. The 's' at the end of leurs is pronounced as a 'z' (usually) if it's followed by a plural word that starts with a vowel-sound, i.e. leurs chapeaux (their hats) = lehuhr chapoo versus leurs enfants (their kids) = lehuhr-Z-enfant
Also, the context is important, which Duo doesn't provide. Use common sense; if you are still stuck between the two, listen to what the person is saying. Suppose you hear either:
Leurs/l'ours manteaux sont rouges
One makes sense and the other's structure is flawed/nonsensical (Unless you happen to stumble upon a bear/bears with red coats - and even then the sentence should be: les manteaux de l'ours sont rouges)
"kids" is "gamin(e)s" - both are more colloquial than "enfants/children". We recognise that the difference between "kids" and "children" in English isn't as wide as that between "gamins" and "enfants" in French, but to help you understnad the difference in French, we only accept "children" for "enfants" and "kids" for "gamins", and vice versa.