As a general rule every employee is replaceable, there's always a backlog of work when people aren't about to get layed off, and there's never a great time to leave.

My advice, just stick to the facts. The reaction may not be anything you expect. Small companies sometimes fire people when receiving notice. Maybe there's a lame counter offer that puts the departing employee on the spot. Maybe there's already a replacement in the pipeline: in a small shop it's often no secret that someone is looking for a job.

Don't burn bridges.

Good luck.