How could I change this sentence in order for it to sound more natural and easier to be understood by a toddler?
I've been using 'unassailable' or 'take them apart' but both sound strange.
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There's nothing wrong with saying "Take apart your legos*† before putting them back into the box." (Or, equivalently, "Take your legos apart before putting them back into the box.") "Take apart" is a fairly common phrasal verb, and it's about as simple and straightforward a phrase as you can get to express this to a toddler — or anyone, really: it's not baby talk or otherwise condescending, it's just a simple phrase for a simple concept.
*American English usage considers the "lego" to primarily be the block itself, and only by normal pluralization (etc) to be the collection of such blocks. British English is the reverse. Since you put "Legos" in the title of your question, I assume you're using American English, which conveniently is my native dialect.
†As a generic term for a type of block toy, there's no need to capitalize it. The trademark is in all-caps anyway: LEGO®.
Another option is to dismantle your Legos.
to disassemble or pull down; take apart:
They dismantled the machine and shipped it in pieces.
No one has thought of explaining why the expression unbuild is inappropriate.
After all, if we do a belt up, (fasten) we can also undo it.
Likewise, you can fix your hair into a bun or ponytail, and later unfix it.
We lock the car door when we leave, and unlock it when we come back.
Yet, to build and unbuild lego (or legos) will sound weird to many native speakers.
If we can assemble an Ikea wardrobe, i.e. we fix the pieces together; we ought to say: unassemble, when its time to move to a new home. But unassembled refers to the flat pack that we have bought at Ikea, the wardrobe that is in pieces and lies patiently in its box waiting to be assembled. Instead, the correct verb to use is disassemble.
English is never 100% logical, why shouldn't a child's construction be unconstructed? The word exists, but unconstructed does not mean unbuild, it means "not (yet) constructed". Well blow me down...
I suppose to unbuild something would be like asking a partner to uncook dinner because it was unappetizing, once a meal has been cooked, it cannot be uncooked. In fact, uncooked food means food which is raw, or has not been cooked.
So, I think the same theory applies to building. For example, when a sandcastle is built, the action is completed. A sandcastle will not last for days, but it might survive for a few hours, until the heat from the sun dries the wet sand and the castle crumbles, or until it is washed away by the tide.
A different action must intervene, in order to reverse the process of building. And, by necessity, it must be destructive. Destruction, not uncreation, is the opposite of creation. The sandcastle must decay; crumble; disintegrate; be knocked down; collapse; or fall apart, in order for it to return to its original state.
A child's lego construction must therefore be destroyed, in some measure, i.e. taken apart if the lego bricks are to return to their original state.
I agree take apart sounds the best for a toddler, but what I would say is "put your legos back in the tub/box".
The toddler legos come in a big plastic tub. If you bought several tubs then you likely store them all in a bit bin or box. Telling them, then showing them putting the legos back in the tub requires taking them apart, is probably better then trying to explain vocabulary to someone that is barely speaking.
That said, I would personally use disassemble in place of take apart. No reason not to start the vocabulary building with simple house hold tasks.