Do you think it's cruel to spend the whole of my children's summer holiday from school travelling?

My husband and I are planning to take the children travelling during the six week break from school. My MIL thinks it is cruel to take them away for 6 weeks, taking them away from their friends and family for the six weeks and that if we want to go away we should leave them at home, even though they are really excited about going. The children are five, three and two. We were thinking a week in Paris, a week in Warsaw, four days in Sofia, a week in Berlin, a week in Madrid, a week in Vienna, and then spending the final 3 days in London. Or maybe just doing Warsaw, Paris and Berlin and spending 2 weeks in each. We live in the UK so there will be no jet lag and we can fly around Europe quite easily or get the train.