Amazing, high-quality inflatable bed that converts an airplane seat into a lie-flat bed for your young child (they might juat have to scrunch up a little to fit!) Recommended particularly for 2-5 year olds. Inflates easily in just 90 seconds (comes with a compact pump). Is allowed by lots of airlines - check the list on the official website to check that your airline accepts it: https://www.flyawaydesigns.com/pages/airlines
We've only used it ourselves once so far (for a KLM flight) so it's in like-new condition.
It is unique in the market and only sold from the company's official US-based website, for $159 + $20 shipping. So if you're not sure about making such a large financial commitment, renting it could be a great option.
Read all about it here, including many 5-star reviews: https://www.flyawaydesigns.com/products/flyaway-kids-bed