I agree that 50 mins is way too tight, but how long is long enough would depend on:
1. How old are the kids? If it was 12,13,14,16, and 2, then that's very different than a 16 year old and 2 year old quadruplets. In the former case, the kids are a net positive (can help move the 2 year old). In the latter, a negative.
2. How many adults?
3. How much gear do you have to transport (which is closely linked to #1).