If for no other reason than convenience, you should get the Octopus card. It also happens to offer up to a 10% discount on the regular one-way fare.

Note that you have to pay a deposit of HK$50 for each Octopus card (children's cards may be lower?) and then because you will only use it for a short period of time, you will pay a handling charge of approximately HK$7 when you return the Octopus card for redemption.

Also, I think most if not all of the MTR stations have elevators from the ground level to the ticketing concourse to the platform(s). While there are also escalators, you will find that they only go in one direction and that there are still stairs you will have to go up/down. Best to take the elevator if you have a pram/stroller but it is a bit of an inconveniece since there are many exits but only one elevator exit usually. The elevator for Causeway Bay is right by SOGO so that shouldn't be a problem. Also, you can probably avoid Causeway Bay and just go to Harbour City/Ocean Terminal by the Star Ferry or the SOGO in TST. In either case, best to go shopping midweek and avoid Sunday at all cost.