Boston has Marriott Custom House. Nothing larger than a 1 bedroom sleeps 4. It exchanges through II. Summer would be a very difficult exchange without Marriott priority. Atlantic City isn't ideal for Philadelphia but it is doable.
For city stays where there isn't a good timeshare, you might want to consider 2 or 3 nights in a nice residence Inn with good public transportation options and actually stay in the city and maybe hook that onto a 5-7 night timeshare stay in more of a resort area in the general area. If you have to commute 1.5 hours in traffic each way it is so much more exhausting especially for kids.