Beware of the assumption that any 20 gauge will automatically recoil less than your 12 gauge. My brother owned a youth model 20 gauge that kicked like a rabbid mule.
We adults invest a lot of time in finding a gun that fits us well, and has the ideal balance and sight picture, and we dress 'em up with special recoil pads and combs, etc. Felt recoil from our favorite 12 may be a lot less than from a given "youth model" 20 gauge. In particular watch out for those light, cheap single shot starter guns. I've owned a couple of them and I call them "shoulder breakers."
Take care to make sure the gun fits your daughter correctly. Find one that mounts and swings naturally for her. Then, take her to the range and make sure she's competent to hit clays with the gun, and feels good about shooting the gun generally. When in the field don't over-choke it the way adults tend to with their own guns. Your young shooter should exclusively be taking close range shots to start out with. IMHO, for a novice hunter you want a 70% pattern at 30 yards, instead of at 40 - unless you're willing to let her water swat, then the tighter the better for "turkey shooting" the sitting ducks in the head and neck.
Also, at 30 yards you don't need the big pellets we've learned to use at pass shooting distances. Just about any size pellet will kill ducks at that distance, which gives you the luxury of using a smaller shot size to deliver a denser pellet distribution and better chances of a clean kill with less choke than you'd normally use. Early season Blue Wing Teal are awesome for providing a youngster their first hunting success, but their vital cross section is less than half as big as a Mallard's. If 90 - 100 pellets in a 30" circle is required for clean kills on Mallards, you should have at least double that pellet hit count for BWTs.
Note: Kent just came out with some really nice 2 3/4" 20 gauge "Upland Fasteel" shells. At 1500 fps, the 7/8 oz load of #6 steel should perform quite well out to at least 30 yards and probably more - if you can find the shells!
If I had an unlimited budget my kids would have started out with 28 gauge 0/Us and Bismuth rounds for duck hunting. I did invest in some 28 gauge chamber inserts to let them shoot the little 2 dram 28 gauge target loads out of 12, 16 and 20 gauge guns. But even at 10 years old my petite daughter found the noise and recoil of 28 gauge target loads intimidating, and she has quit shotgunning for now. If I had it to do over again I'd have made her wait a year or two to start shooting anything heavier than her .22 rimfire (which she loves and is very accurate with).
Of course, her twin brother let nothing deter him and pretended not to feel any recoil even from loads I knew were a bit heavy for him. Now he's 13 and shoots a youth model Remington 870 in 20 gauge with a special recoil pad. Last time we shot clays together he beat me! For ducks this year he used 1 1/8 oz Hevi-Shot loads. Last year it was 1 oz 16 gauge Bismuths out of a classic single shot model. Between the two years he has taken about a dozen waterfowl and fired perhaps 20 rounds. Thus, it has been quite economical to shoot the denser shot types, which can put the sub-gauges on par with steel out of a 12.
These have been my experiences.