What should we do concerning IP created by freelancers?

standard employment agreement should contain assignment of copyright over output supervised or managed by yourselves. There may be carveouts for infrastructure (eg libraries, open-source) or out of business hour projects (pet hobbies) but you have to be careful as some jurisdictions look at the contract to see whether you are actually disguising employees as freelancers.

Without knowing what jurisdiction you are in, the question is difficult to answer. As for patent ownership in the USA, you would need to have an attorney look at the applicable State Law and probably in light of that State Law address ownership in a contract. In the USA, if you intend on maintaining the code as a trade secret, then you would need to have a lawyer look at the applicable State Law. In the USA, copyright ownership should be addressed in a contract by your lawyer. Also, in the USA, registering the computer code as a copyright may also help. All of the above depends on what open source code your freelancers are using. You should keep your ideas regarding the project secret (i.e. do not tell the freelancers) until you consult with a lawyer. The above comment is not legal advice and does not constitute an attorney-client relationship. You should consult with a lawyer as soon as possible to review your issues.