I think you'd have an easier time monitoring your child in a window seat if they were in the row in front of you; so say if you did select 8E, you'd be better able to visually see what they were up to if they sat in 7A without having to get up (or at least see they weren't running around).

I can't really speak to the lavatory noise for the middle seats in this configuration, but the one time I chose a last row window seat in business near the lavatory I didn't have issue with the noise, but there was some foot traffic that I'd prefer to avoid if I could.