First of all, it sets up standards for kids about what is ok and what isn't.  If they wore a bikini when they were 2, why can't they when they are 20?  And if you think it is ok for a 20 year old to wear a bikini, please read Let's Talk About Modesty and also the Modesty Test. 

Second, there are perverse men out there who are watching your little girl.  It is disgusting to think that a man might be turned on by your little girl, but if you dress her in immodest, flirty, or revealing clothing you are putting your daughter out there to be viewed.  I am not justifying the thoughts of such men, but why tempt them or even step in their path?  Why not dress your daughters as proper young ladies and not young ladies who are conforming to what "everybody else is doing"?

Third, and this kind of goes along with number one--the more skin a little girl reveals, the less awkward it is for her to reveal it later in life.  Whether it be with the prom dress she chooses, the bathing suit she chooses, or just plain undressing herself in front of a young man whom she is not married to.  If it was once ok to wear a bikini in front of a whole crowd at the beach, why wouldn't it be ok to wear only a bra and underwear (really, what is the difference?) in front of a boy that she really likes?  You might be telling her that sex before marriage is wrong, but what messages are you sending her by what you allow her to wear?

Fourth, what messages are you sending to the boys around your daughter?  Boys are tempted by the clothing that the girls around them wear, and the sooner those temptations begin, the more likely they are to lead into sin.  Do you really want your young girl to be a stumbling block for a young boy?

Please be thinking and praying about these things.  I do not have any daughters (yet), but if I did, these would be my house rules:

1.  No belly shirts or shirts that reveal any tummy or back at all.

2.  No low rise jeans or jeans that reveal the underwear.

3.  No sexy underwear at all.  A little girl does not need cute, fancy underwear (or bras for that matter)--who else is going to see it? 

4.  No sleeveless tops.

5.  No short skirts or shorts. 

6.  No shirts with logos or writing across the chest.  Especially the shirts that say things like "princess" or "cutie" or "spoiled" across the chest.  They draw attention to the area (and even if there isn't anything there yet, it still sets the young girl up for future problems), and those messages just scream "look at me" and portray a certain type of girl.

7.  Absolutely no clothing that makes a young girl look like a "little woman."  A little girl is to look like a little girl.

8.  If we ever have a daughter, I will apply the same rule that Randy Alcorn has mentioned he used in his home--Daddy has to approve what clothes his daughter is wearing in public.  Daddy knows what young men will be tempted by, and he will quickly (if you submit and give him the power to do so) let you or your daughter know what is inappropriate and ask it to be changed. 

9.  Ultimately, as a little toddler I would begin instilling values in her that I would want her to carry with her for the rest of her life.  Even though a little girl doesn't have much of a chest yet, I would not allow her to run around without a shirt on or take baths with boys, or get dressed in front of boys, or flip her dress or shirt up.  These are behaviors I would not want her to have as a teenage girl, so why not teach the word modesty when she is young?

As a side note, I recommend checking out the Modesty Test and thinking about what it is you are wearing.  Your little girl is watching you and learning from you.  What do you wear at the beach?  Out to a formal dinner?  Many women who appear modest most of the time tend to wear more revealing clothes out to a nice dinner or to the beach.  Keep in mind that your little girl is watching you.