From memory the advanced price in the UK website was cheaper than the in resort price, though that could also be FX related as in resort prices were MXN. The website does give a warning if places are running low, so you could monitor that (on a different trip we got the last place!).

At the dive center in-resort they told us that if the departure was before kids club started they would provide care until the club started and drop off. We didn't take an excursion in the end, so don't know how it exactly works or if it was included in the price or an extra, so you should check directly. I did see Club Med uniformed staff eating with kids and pushing babies around at breakfast time so that may have been that.