We looked into the booster seat option when we had a child of relevant age, and gave up because British airlines don't seem to comprehend their use and don't publish dimensions anywhere. Similarly, seats aren't sold as being airline-friendly. However, we do get those nice organge lap belts, which US airlines don't supply, but of course you do need a suitable adult lap for the child to sit on, which brings us back to the original problem of one adult, two small children.