Welcome to HealingWell! Sorry to hear about your diagnosis of IBS but there are a lot of knowledgeable people on this site that can help you find ways to relieve your symptoms. Constipation, bloating, gas, nausea, and stomach cramps are very typical symptoms. I personally am prone to excessive diarrhea and don't have many other symptoms anymore, but here are some suggestions that might help you:
1) Taking probiotics in the form of yogurt with active, live cultures (like Lactobacillus and Bifidobacteria), or as a supplement. These "good" bacteria are naturally found in your body and taking a supplemental dose each day can have a profound effect on IBS symptoms. Sounds like you live in the UK, so I'm not familiar with the brand names over there...but if you ask the grocer or pharmacist about where to find probiotic supplements, they'll probably be able to help you.
2) Increasing the fiber in your diet. You should aim for about 25-30 grams of dietary fiber daily. You can try taking a supplement, like methylcellulose, which might help as well, although you should remember to slowly increase the amount of fiber you're taking in because too much at once can cause excessive gas - which you don't need any more of!
3) Regular exercise can help move things along in there. Aim for 30 minutes, five days a week. Sounds like running after your children might be helpful, if they're fast enough
4) Keeping a food diary so that you can track your symptoms and see if any patterns arise with certain foods. Noticing patterns is very important for IBSers!
Best of luck; keep posting and let us know how things are going!