re-negotiating a start date

[paulahill]

Last week I accepted and signed an offer with a starting date of April 24 (for hospital orientation) for a specialty training program that starts May 8.  The following cohorts begin May 29 then June 26. I am having issues with childcare and I want to negotiate the start date to June cohort. I feel like it is so far out but it would make my life so much easier with childcare. My mom was slated to retire at the end of March but they haggled her into staying until May to continue training her replacement. Which of course happened after I already accepted and signed the offer. She was my childcare plan! I'm sure I could find hired help but these are my kids and I don't trust anybody like my mom. How can I approach requesting an extension on the start date? Do I go to the recruiter? HR? Am I going to blow this opportunity by asking this? I need advice so bad.