thinking back to lots of other threads on here the best advice is to book your 10 month old a seat. it gives you much more room and the sky cots are for really quite small babies.
the whole trip will be a lot more stressful if one of you has a baby on their lap for such a long flight. the MN wisdom seems to be book the extra seat and take the hit on it. at least your little one can be laid down to sleep.