Honestly, if it were me, I would take this shoot outdoors.
First of all, the weather is great and you should take advantage of it. Not only will the baby be fussy indoors in an itchy, annoying dress (I could be wrong about this, I've never worn a dress), she will get impatient and try to grab the equipment. Whereas outdoors she won't be easily distracted and you can have her attention by having your wife assist you with the baby.
Second, think of it this way, green grass and a tree with your daughter sitting on the ground or standing in front of a bed of flowers with Easter eggs randomly all over the ground. The scenario is much more realistic than a white background shot.
And lastly, to answer your question, you would probably need only one flash and without any modifiers... maybe a CTO gel, if anything. You can leave the stands and umbrellas at home. Just ask your wife to hold the flash and aim it at your daughter.
This is how my 'day 65' shot was done.

All I used was a circular polarizer filter on my lens and a flash. Stop down the exposure a little so the background can look bluer and darker. Use the flash as fill light on your daughter. That's it.