Bit of a silly question methinks but how old were your DC's when they started drinking tea (black with a little milk). My DD is 21 months and is constantly asking for a cup of tea when I'm having one!! She is getting a tad board with water and milk...... I'm sure I remember drinking tea with my Nan when I was about 3.
Sorry- the tannins reduce absorbtion of iron from food sources if you wish to be pernickety- which is fine in healthy adults, but obviously more of a problem in children who have smaller stores of iron.
I've been giving 'tea' to DS since he was 2 because he wanted to join in with me so I dip my used teabag into a cup of warm millky water. Hardly any 'tea' in there really but he loves it for whatever reason :-)
I have to agree with swanker, certainly for the younger children, ie under fives. Drinking a lot of cow's milk can also lead to a problem with iron deficiency, admittedly, it is unusual but does occur.
We drank tea and even drank very milky coffee as kids, but not till primary school age. I'd suggest some redbush/bush/rooibos tea, which looks like black tea and tastes similar to it. It can be drunk with milk just like black tea, but is naturally caffeine free and low in tannins.
Hipp Organic used to sell the fennel tea MmeLindor mentions above - this was 3 years ago, but they are probably still doing it. It is also supposed to be good for digestion, heartburn, and colic in babies (we gave it nearly cold in a bottle to DD1 when very small and windy).