I have found in the past it always helped to be prepared for the situation that can potentially be a trigger.- don't just wing it yunno? It is okay if you need to step outside for as long as it takes for you to take a deep breath and talk yourself through the situation. Also, can you tell your family that is going to the restaurant that you are feeling concerned that can stand beside you and give you there support- it also helps to keep you accountable if you tell everyone that is going to be there that you aren't drinking, that you on Day 2 and that you are at a time where you could really use some accountability by your family. Also, is it at all possible to ask those who are going to the dinner to not order alcohol? The dinner is for your son and he is not old enough to order a drink so it isn't like you would be putting a damper on his evening since he won't be drinking anyway.
If in the end you really feel like you are not able to go to the dinner without drinking then remind yourself that it is okay to not go to the dinner. I am sure that if you were to tell your son why you can't go- if you are honest- will not ruin his day. Any child would rather have a parent that is sober and not coming to dinner to continue their recovery then to have a parent that is an alcoholic drinking and potentially becoming quite drunk and not there for him.
You can do this! Just take it hour by hour if that is what it takes or take a step back and decide if this is a safe situation for you and if not, that it is okay not to go.
Your recovery ALWAYS comes first because if it doesn't, all other aspects of our life go to hell.