The little girls are called bridesmaids; I think the little boys are called pageboys. Sometimes there are one or two older ones who help with the train of the dress, or there might be only one who's called I think the chief bridesmaid, as at Prince William's wedding. That's in the UK.
In my experience (which may reflect AE usage), the little girls are called junior bridesmaids. A bridesmaid, without the "junior," would usually be a friend or relative of the bride. She is typically more or less the bride's age, not a little girl.
We have a US/UK difference at work here. In the UK, it is common for a bride to have a group of child attendants, and the girls would be called "bridesmaids". In the US, bridesmaids are usually the bride's adult female friends, and the only little girls one will find among the attendants would be "flower girls" who lead the procession.
Note that the part of the wedding dress you are thinking of is a "train", not a "tail", and it is common to let it be drawn along on the floor after the bride rather than to have it carried.
In NZ we'd also call the adult women bridesmaids (with the principal bridesmaid perhaps known as the maid or matron of honour, depending on whether or not she is married) and the little girls 'flower girls'.