Good Advice.
I think with the issue of your DH's leave and childcare issues, and the administrative tasks that you will have to spend your time on - that it seems more practical for you do go on your own.
However, it seems from what you've said it's a question of you worrying in advance that you will feel lonely and emotional and bereft and you worry that the visit will trigger grief and you may want some emotional support with you for that.
Have you considered talking to a bereavement counsellor about this and your GP ? To help you prepare for the trip?
Are you worried that you might break down and cry in the offices? You are dealing with bereavement and they will think nothing of you taking a short break to calm yourself. They've seen it all before and will do their best to be kind and help you get through it. It's in their interests to do that. I once was in a difficult situation and to my great embarrassment did break down but they were so kind and nice that we managed to get through it and get on with the task in hand.
Given the practicalities, having DH and DS there, hanging around, bored and waiting, might make it more difficult in some ways and harder for them to be the emotional support you want in some ways.
Maybe it would be enough to be connected to your DH and son on the phone/facetime, having talked to your DH in advance about what you need? Just Knowing that he's going to be available may be half the battle? Have you got any friends at homeyouu can also be in contact with, who would be willing to listen or call you once or twice during the week - that way you would still feel connected and less alone.
It might even be nice to take pictures of the city and places you do remember so that you can send them to DH and DS as a lighthearted casual talking point to start off with rather than just having a conversation that you both worry might be along the lines of "Hello now I'm going talk about deep emotional issues" - it might be less daunting for you all to have something relatively lighthearted to kick off the conversation and help normalize the situation?
I also think if you took the advice from pp above, about planning nice things to do, when you are not in solicitors offices - there are always interesting places to visit in a big city, nice restaurants etc.. and build yourself a full schedule, take some guided tours so you are not wandering on your own - you would feel less lost - because you'd already got a list of places at your fingertips where you could be. Take some good books, load your ipad/phone with some good "episodes" and keep yourself distracted in the long evenings. .
You may even end up feeling by the end of the week as if you've climbed a difficult mountain. You only have to climb once BTW- once its done, its done, you will never have to go through that again - and come down the other side lighter, knowing that you made the very best of a difficult situation. And you may find that the trip, whilst bringing up sad memories may be a form of closure for you and help you feel stronger.
Then you can go home to your lovely DS - knowing that you've got it all off your plate and plan a lovely treat to look forward to and celebrate your return together. I know that having something like that planned would help me get through a difficult week.
Wishing you all the best OP.