If your sprog is under 2, they should be able to reserve you a seat with a bassinet in front for them to sit/sleep in. These are the seats at the front of the cabin and you can get either a cot or a something that looks like a car seat. My wife and I took our 10 month old all the way to Sydney and back in one and she was quite happy, try calling BA and making sure you have a bassinet seat. What seats are you in currently and on what aircraft?

On the upgrade thing, yes to upgrade with Avios from traveller you need one of the higher fare buckets. If the difference in fare to got to WT+ is only £400 I would jump at that. If it is a simple fare difference and not a special offer you might be able to use your Avios to get into club.