"The children have a turtle."
Translation:De kinderen hebben een schildpad.
In this case it's easy: all plurals use 'de', hence it's "de kinderen". Words ending in -tje/-je use 'het' - 'het fietsje', 'het kindertje' etc. All other words you'll just have to learn by heart, unfortunately. If you're not sure which to use, you can doublecheck using a dictionary like Van Dale.
"kindertje" is not a Dutch word I'm familiar with. I suppose you mean "het kindje?".