"They were thinking about their children."
Translation:Ils pensaient à leurs enfants.
penser de means "to think about/of", in the sense of "to have an opinion about"; e.g. "What did you think about that movie?" ( « Qu’est-ce que tu as pensé de ce film-là ? » )
Unfortunately there is not, especially when it comes to verbs that can use either preposition to change their meaning. It's best to learn these verbs individually with their preposition.
- penser à = to imagine, to consider
- penser de = to give an opinion about
In a sense, English is the same way. Take, for example, the verb "to get." Add a preposition and you get a lot of different verbs: get by, get along, get around, get to, get from, get in, get out, get for, get off, etc.
I would like to know this too..because it seems to me that "de" would have worked in this sentence.
What's wrong with" Ils réfléchissions à leurs enfants."? Can't réfléchir be used for "thinking about"?
I'm not sure if réfléchir could be used in this situation, but it would be "ils réfléchissaient."
Because that's not the imparfait/imperfect tense. That'd be passé composé and implies the action happened and then ended. The imparfait ("were thinking") implies the action was a continuous one. They kept thinking about their children and not just thought about their children for a second then stopped suddenly.