I actually googled the idea of writing prompts for songwriters and got some funny ideas. They may be helpful to that solo singer/songwriter who expects to perform his own songs, but not very useful to me. One of the ideas was to write about a bad break-up. A sad love song always seems to sell well. (A lot of people can relate.) Or write about a relationship you look forward to. (A lot of people can relate to that, too.) Or take any noun and try to write a song about it. (Really?) Or do a Public Service Announcement about some important subject and write a song about that. (Interesting) Of the 100+ ideas, I really did not find any that were prompting me to write a song.
Then there is the melodic side of songwriting. What can you do to get a melodic prompt? Which comes first anyway --- the words or the tune? For some songs, the beat, groove, or rhythm obviously needs to come first.
Knowing the type of song you want to write helps.
If you are writing for CHILDREN, definitely don't succumb to waxing eloquent. Remember that simple is best for kids. Think about a child's singing range and the intervals such as a descending minor third that little children sing naturally early on. They like songs that explore the things they encounter in their world. Definitely NO romantic LOVE songs.
When writing for Children's Theater, the story comes first. It also helps to follow other tried and true conventions such as writing an Opening Production Number, or writing a Message Song that also becomes the Finale, a few Solo Songs for the lead characters enlightening the audience about their motivations, a couple of Small Ensemble Songs that move the plot along, and the occasional Comic or Dance Songs just for fun. I find that researching historic musical styles is very insightful. Obviously, finding or developing a great subject is key when writing Children's theater or any theatrical piece for that matter.
Though writing prompts can be useful, they might not be what is really lacking. Desire. Drive. Determination. Those are the real tools you need. You definitely need to have a DESIRE to get started, then you must summon the DRIVE to keep going and the DETERMINATION to finish.