Hello, I can recommend a nice review that goes into iron, including nutrient-nutrient interactions, found here. In summary, we probably need to be more concerned when we're thinking about calcium coming from supplements (high doses) rather than the naturally occuring concentrations in dietary sources. Whilst calcium from food and drink does inhibit iron absorption in theory (though the mechanisms are still unclear), there is very mixed evidence out there. Some studies tried separating out iron- and calcium- sources of food to consume at different times of the day, but this didn't seem to do anything (i.e. iron absorption stayed the same whether the food items were consumed together or separately).

What seems to be a more evidence-based solution is to include a source of Vitamin-C with the meal if possible, as this will overcome any small effects of the iron absorption inhibiton caused by the calcium. So where resources allow, some fresh food juice, some green leafy veg etc....these food-based sources of vitamin C will help the iron from plant-based sources be absorbed despite the presence of calcium in the meal.

I think we also need to be careful of how much information we expect mothers to take on and implement - if it's easier to prepare the porridge with milk (and if the child prefers it - another key consideration!) then I'd say the evidence base is not strong enough to insist on having the milk at a different time.

In summary, the literature is very mixed on this issue (see another recent review here). In areas of high iron-deficiency anaemia maybe we'd want to look at other strategies to improve iron intake. 
 

@ Philip James, Thank you for the response and  informative articles.