How far in advance can i make this icing? Any concerns i should consider if using this for a 2yr old's birthday cake?

Swiss meringue buttercream is actually quite easy to make, as long as you don't overlook a few key points of no return:
1. Be sure your butter is at room temperature, not warmer, not cooler. If too warm, it will melt rather than emulsify, and if too cool, it won't emulsify well.
2. Whisk the egg whites and sugar constantly over simmering water until they reach a temperature of 140 degrees.
3. Whip the meringue on high speed with the whip attachment until the bottom of the bowl feels COMPLETELY cool to the touch. If too warm, the butter will melt rather than emulsify.

You can make the icing well in advance, as it will hold perfectly in the refrigerator. Just be sure it is tightly covered, so as not to take on any odors from the fridge. To reconstitute it, scoop out 1/3 of the icing and melt it in a stainless steel bowl over sintering water. Transfer the remainder of the (cold) icing to the bowl of your mixer. Pour in the melted icing, and using the whip, begin mixing on low speed, gradually raising it to high as the two begin to blend. Once they have, whip the icing on high for a couple of minutes to reincorporate enough air to make it as spreadable as if it were freshly made.

Have a wonderful party!