Similar bag to Lowepro Slingshot 300

Have previously owned this bag and need input from you guys.

When I get my Sigma 100-300F4 and 1DII, I will need a bag for an unusual situation.............​.

1. The lens will always be attached to be ready for shooting.
2. I always carry my baby son in my arms (he's a very big 9 month old, so can't use slings). I don't do pushchairs.

My wife manages to carry him for a few mins at a time whilst I take photos.

So.......I need a rucksack type bag that is just big enough for body/lens only - but one that is easy to access my camera from. Obviously my son comes first, so if it isn't practical the photos can wait until he is older.

Anything out there that is cheaper (OR BETTER) than a slingshot 300.

The Lowepro Fastpack series is backpack-style--two straps instead of a sling. It has a side entry feature that can be used by sliding one strap off the shoulder. The Fastpack 100 is fairly small and might meet your needs, with a little room for extras above the camera compartment. I just got the 250 model, which is about the size of a daypack.