Translation:These salads are very light and we want to eat them.
In a situation like this with two verbs next to each other and one direct object, I prefer the alternate form of "Estas ensaladas son muy ligeras y queremos comerlas." which has the same syntax as in English ("... we want to eat them") as opposed to "... y las queremos comer ("... [them] we want to eat").
It is entirely normal in English to talk about a food being "light," even when people are not talking about or thinking about being on a diet, as someone else said here. Someone might want a light meal late at night or a light lunch, because they have big dinner plans later. "Light" and "heavy" can be used to refer to any one dish, or to a whole meal. Super-normal English, and Spanish too!
And we use heavy in the same way (even in descriptions: heavy cream is one that pops immediately to mind.) I constantly say "I want something light" not b/c I'm on a diet but b/c I don't like heavy/rich foods. They make me feel weird. (So I should memorize this phrase)
Is there anything wrong with saying "Estas ensaladas son muy ligeras y queremos comerlas."? This is how I would say it, and how I've heard constructions like this from native speakers in more than one country. Seems less stilted and more natural, but perhaps it varies from one country to another?
If you don't get the right tiles for the sentence you imagine, Duo is expecting a different answer. Those tiles are automatically generated from one accepted answer, which might not be the one you have in mind.
Since you had comer and las separate, the machine apparently expected you to write "las queremos comer".
I really hate to complain but it seems like with this program i am spending a massive amount of time learning phrases that i will never use in real life. Perhaps for increased effectiveness the sentences use should be ranked by frequency of use case in a real life scenario.