It is perfectly safe to breastfeed your child if you're lactose intolerant. It does not put them at greater risk of becoming lactose intolerant.
If your child is lactose intolerant, they may be able to consume small amounts without experiencing symptoms. This is quite safe, but you may need to experiment in order to establish a comfortable threshold.
In some cases, your child may not be able to tolerate any quantity of dairy food at all. If so, your doctor can refer you to a dietitian for nutritional advice. There are also lactose-free soy formulas available for babies, while soya and nut milks make good substitutes for dairy milk, so older children can drink these instead. Fish, tofu, nuts and green vegetables are also calcium-rich and lactose-free.
Your doctor or dietitian may recommend the use of lactase drops. These can be added to milk to "predigest" the lactose in it, making it safe to drink.
Remember that secondary lactose intolerance, brought on by a bout of illness such as flu or gastroenteritis, is only temporary. Avoid feeding your child dairy food during the illness, but a few days after symptoms have passed you can gradually reintroduce dairy into the diet.