Most carriers have a discount for a child under 2 years of age. Children over the age of 2 years are issued at the full cost as for adults. These are the conditions of airlines and hotels!
Yes, children can be taken to Umrah. We recommend that you refrain from doing so if possible. Umrah is not rest, but the worship of Ibadat, which requires the application of some minor, but still effort. A trip with children will be somewhat burdensome for both adults and children.