ah, that's really nice, floatingonthemed - I bet your friend would be so touched to hear you miss her baby more than your days off.
I am all for some free time from toddlers to go shopping. I well remember my toddler son's eager anticipation of a session at the Bluewater shopping mall creche. He loved the toys, the company and the new surroundings and as soon as his pushchair straps were undone, he would make a run for the entrance. He would be even more devastated then me if the creche was full up 
He knew what he wanted, I knew what I wanted. He probably knew I didn't want to take him shopping, but didn't seem at all upset about it.
The good thing with creches at shopping malls is that you can also have some time with your children before and after the session - go to a nearby soft play centre, playgound, into a few select shops,see a film, stop for someting to eat together, so the shopping trip isn't all about you shopping.
Now my son is 6, one of our days out involves going to The Glades at Bromley, having a swim at the pool there, grabbing a snack and then ds goes into the creche for a couple of hours while I do the boring but necessary bits of shopping. I did this with him when he was much younger, too.
The only problem is the cost - that's what makes me feel guilty and nothing else.