I am a dentist, and would never prescribe a muscle relaxer as a first line defense to tooth grinding, especially someone who co sleeps and breastfeeds. Please, try and have a nightguard made, or make one yourself (not as good, but will work, although those can sometimes make matters worse), before using the muscle relaxers. I also recommend to my patients to be aware during waking hours of their grinding and clenching. If they grind to place their tongue between their teeth (you won't grind on your tongue). Before bed, to do some relaxation techniques, tell yourself not to grind at night (the real kicker on that one is that you have to actually believe it will work) and to try and not allow yourself to be stressed before bed.
Our teeth are only made to touch, and if I remember the percent correctly, less than 1% a day and that is only when we swallow. Our teeth are not meant to touch even when we are eating since there is supposed to be food between our teeth. So if you are clenching, and grinding you are actually overusing the muscles that put our teeth together, causing them to fatigue and in the long run, making the problem worse.