Purchasing a seat in Euro Traveller can be a similar amount to Club, and from experience can involve several hours on the phone to BA (no exaggeration). Obviously this applies to two adults travelling together with an infant, otherwise it wouldn’t be fair to assume the use of the whole middle seat if only one adult.

A few things for the OP to note from my experience. Not all cabin crew know how to stow the centre table, so don’t count on that happening.

By the time our little one was toddling I preferred the extra space of row 1. They had always wanted to sleep on us and hadn’t been keen on sleeping across the seats so the fixed armrests hadn’t been a big deal. Of course YMMV.

There should be some pillows and blankets available, which can be useful for naps. Ask the crew and if they are unsure they are usually in a blue bag in the overhead lockers (if they’ve been loaded).

Finally beware all toddler water cups the pressure changes tend to make them spray water everywhere when opened in flight.