"There is love inside you!"
Translation:יש בתוככם אהבה!
I hope someone responds to your syntax question. The prepositional phrase בתוככם goes before the noun. I'm not sure if that's idiomatic or if the syntax is somewhat more fluid. Your sentence doesn't work for other reasons, however, including that with אהבה you would need fem. זאת and the preposition has no object--it's just the base preposition. So your sentence makes no sense grammatically even though your question is good.
It's a good question, but see above. Unfortunately, we have received no reply from a DL Hebrew moderator on this matter. It may be that's it's an old question and so in a new iteration of DL Hebrew they will accept the singular when it's a simple translation exercise from English to Hebrew.