They only have to pay you the greater of SMP (for that week) or the KIT day pay - so you can end up no better off for having been into work, but some places (not where I work!) will give you the KIT day pay on top of SMP.
So basically you can end up worse off for having been into work by the time you've paid for the commute and lunch, and that's assuming your childcare is free. 2nd time round I didn't even bother filling in the paperwork for KIT days as it it was pointless (I just went in for a couple of team meetings and stood around chatting for half a day - literally to keep in touch).
(I think before KIT days were introduced you lost your week's SMP as soon as you did any work at all, hence could end up significantly out of pocket for doing a couple of hours work. The current system means you should at least take home the same amount, but in reality you'd have to do quite a few hours/days in one week whilst receiving SMP to end up significantly better off in the financial sense. But I took the view that it was worth showing willing with management by turning up every 3 months or so.)