Husband and wife are traveling for Umrah on May 26th from London and we have twin who are 20 months old. We both need to wear the baby "back" carriers that are worn like backpacks since Masjid al-Haram does not allow baby strollers/pushchairs. If we wear these, does it affect anything about our Umrah or one's Hajj for their Ihram? I am a bit scared as its a long flight and its our frist time.

No there is no problem in wearing bab back carriers since they are one of the prohibted types of dresses in Ihram.