Try and get referral to physio from your doctor - there are exercises that will help strengthen the muscles and close the gap, but you need a proper assessment to see how much separation there is.
Traditional abs exercises, twisting work and sitting up from lying without using your arms will all make it worse, so avoid them like the plague for now. While you're waiting for any help from the GP, you should try and engage your muscles all the time - lifting/pushing a pushchair etc try and bring your tummy button in towards your spine (without lifting your shoulders), as if you're 'bracing' your tummy muscles. With a small degree of separation, and no active use of your tummy muscles you risk putting your back out especially lifting babies that quickly go from tiddlers to being heavyweights!
It's not really like a 'hole' in the muscles, the fibrous tissue that holds the two sides of the rectus abdominus (six pack)together thins and stretches to allow room for the baby. It takes both time to heal and correct exercises. Occasionally people do get an actual gap between the muscles which can cause a hernia, which is operated on routinely by the NHS. You'll certainly be able to get physio on the NHS in any event.
The tummy button to spine technique won't make the separation any worse but sadly you won't see any results for about 6 weeks, you really need to stick with it. Wish I had a magic wand, sorry!
To self assess the gap in your muscles:
Lie on your back, knees bent up, feet flat on the floor.
Place two fingers just above your belly button.
Raise head, neck and shoulders just off the floor to gently engage stomach muscles.
You should feel the two sides of the muscles squeeze on to your fingers. 1-2 finger widths is perfectly normal.
Good luck with the doc, don't worry it's very very common and perfectly treatable with the right exercises and if you have a hernia, they'll fix you up no prob.