I can't speak for Iberia Express (although given my experience of Iberia crew I suspect they won't be bothered by anything that doesn't mean extra work for them). However on BA we often travelled in club Europe when our little one was an infant on lap, and the crew never had an issue with him being put in the middle seat, as long as he was securely on a lap for takeoff and landing.
Did they mind you removing the middle seat table (or indeed offer to do it for you), or was this before the new style seats?