The context can require a particular choice that will carry a nuance, but ever so small that I believe it hardly possible to make an argument about its value. Usually there is no nuance.

• Je prépare un bon petit plat pour mes enfants, pas pour elle car malgré tout le mal que je me donne elle dénigre vicieusement ma cuisine.

"À" would not be quite as expressive, and possibly not quite idiomatic; it is not a preposition that communicates its semantic function as vividly as others, this being due, at least in part, to the great number of functions that have been piled up upon it (TLFi). Therefore the idea of someone being a beneficiary is made more salient by the use of "pour". I would not argue against the use of "à" in that sentence but I definitely prefer "pour".

Anyway, let’s notice that "à" is not at all proper in front of pronouns when use in this sense (pour).

• Je les prépare pour elle. Je les prépare à elle.

This shows again that this preposition, in some of its uses, is lacking somewhat in specificity, in clarity as to what it communicates.