There shouldn't be any API or webservice that does this... because it is a "Bad Idea"™... you're essentially broadcasting your private keys to an online service.

If you need to be able to convert from a seed mnemonic to your private/public keys and addresses, I suugest that you read up on BIP39: https://github.com/bitcoin/bips/blob/master/bip-0039.mediawiki

It even has a list of implementations for a whole variety of programming languages at the bottom of that github page. I can almost guarantee that whatever language you are currently using for your project, already has existing libraries to convert from seed mnemonic to keys/addresses, so you will be able to do the conversion locally without needing an external service.