Dear Naziya,
First of all please confirm whether your establishment is coverable under the Act including these contract labour also. Secondly, check up whether the canteen contractor as an establishment is covered under the ESI Act. If the contractor is not, you may not be able to force him to register his establishment. However, as clarified by Mr. Harsh Kumar Mehta, after including the contract labour, part time hamals, etc., if your establishment falls under the coverage, please register your establishment under the Act and remit the ESI contributions of all employees including the contract labour. However, you may recover the employer and employee contributions from the respective contractors bills.
Once you cover under the Act, you have to remit the actual contributions as prescribed under the ESI Act and not as miscellaneous or labour charges, and comply with all other statutory requirements such as maintenance of documents and filing of Returns.
If the contractor's establishment is coverable taking all his employees deployed in his establishment and in other Firms on contract, you can insist on compliance under the ESI Act by him and seek documents thereof as a precondition of passing of bills.
With best wishes,