I'm trying to go from a full time position at work to a part time position with fewer responsibilities, but is in the field I'm interested in. I am overqualified though. I've been married 3 years. Are they going to assume I want to go part time to have kids and not give me the job? What slant can I take here?
I would just explain that you realize you are overqualified, but are willing to take a lower level position than you are currently in to move your career in the direction you would like it to go. I would just focus on what you could bring to the table for that position and why you are the best candidate -- don't mention the full to part time transition, and if asked, just re-explain that you are "willing to take a lower level position than you are currently in to move your career in the direction you would like it to go."
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"Fashion can be bought. Style one must possess." ~ Edna Woolman Chase
In a cover letter, you don't mention that you're married and you have or don't have babylust. You don't mention your age, race, gender, or anything, frankly - other than how you propose to serve the needs of the employer.
This stuff is always about what you as an applicant do for me as an employer - I have a niche to fill and you're supposed to tell me how you do that better than someone else. In your case, you're basically saying you're going to give me pro capability for less money, no benefits, etc. An employer would be stupid not to be cool with that.
But I need more information - why can't you ask your current company for a part-time/flex-time schedule? What industry are you in that employers aren't wanting more for less? What are you trying to accomplish in the hours you aren't working?
I have to guess that you're seeking a better work/life balance, and if so, that's what I'd express. Say you have so many career milestones already and you've proved how valuable you are, but you are seeking a schedule that is more humane and respectful of your personal life.