I can't do it in two child-proof sentences, but I'll try to come close.
- The EU is ana union of 28 states which has been granted some but not all powers of a central government by the member states.
- UK voters decided to leave the EU. This is complicated because the EU had so many powers for so long and the UK voters told the UK government to negotiate the details with the remaining EU states.
- If the UK government and the remaining EU states cannot agree on the details of leaving, the UK will leave without a deal. That means medical drugs approved in the EU are no longer legal to use in the UK and the other way around, UK citizen cannot travel and work in the EU without a visa and the other way around, UK banks cannot sell products in the EU and the other way around. (There are emergency measures to handle some of this. UK and EU citizens won't be deported March 30th even if there is no deal and no extension.)
- When UK voters decided to leave, they had a simple question with a "yes" or "no" answer. Probably most did not want to leave without a deal.
- The UK parliament cannot agree on if and how to leave, but they do not want to leave without a deal. The UK parliament has a complicated question with many possible answers, and no answer has a majority for it.
Eleven sentences, not entirely child-proof, and admittedly biased against the current UK parliament and government.