Fully understand your dilemma, for myself I would always try and find a way to keep working particularly as you like your job and company, but obviously it's personal preference... for me even though the cost of childcare is eye-watering and may almost or completely wipe out your earnings in those early years, its worth it for the benefits including paying into a pension, keeping a foothold in career (bearing in mind the years when the DC are tiny and needing a lot of care are few in the scale of your overall working life) and psychologically/self-worth I gain from working. But of course there are arguments to be made in the other direction too. If it helps I would try and mentally assign the money spent on childcare as part of the cost the family as a whole has to pay for your DH working crazy hours and having a long commute, not as a direct consequence of you working, if you see what I mean?
Couple of questions/things to ponder, feel free to not answer but would be how my thought process goes!
-What are your current childcare arrangements and could they be changed to facilitate you working more easily? 6.30 is a very early start, I would assume not many nurseries/childminders take them that early. Would a nanny be an option for you? Obviously £££ but would mean you can leave the house before they are even awake if needed, keep them in their home environment etc. If you can't afford that but you have room in your house, perhaps an au pair? S/he shouldn't be in sole charge all day so you'd still need other childcare for the 3yo on your working days but the au pair could drop off/pick up, supervise homework etc. for the 8yo and, depending on how many hours that comes to, do some light housework as well? Sharing your home with an unknown young person is not for everyone but it can work out well all round.
-Is it absolutely necessary for you to get in at crack of dawn just to get a desk? What would actually happen if you came in at a normal time and there was no desk? My work is also hot-desking and recently a bit of 'getting in early to get a desk' brinksmanship has been creeping in, with 'normal' arrival time slipping earlier and earlier as desk-based paranoia sets in, one colleague now regularly arrives at 6am to secure her preferred desk, it's all a bit 'towels on the sun lounger'. I've now opted out, I arrive when I arrive (flexi time), often 9.30 or even later, yes sometimes there's very few desks left (only the un-favoured ones next to the printers or similar) or none at all, if the latter then I grab a vacant meeting room, typing booth or even sit at the staffroom table with my laptop until one becomes available, it doesn't kill me (just need to be a bit careful about posture hunching if not at a proper desk). There are always desks free after lunch anyway as the 'early start' brigade start to go home. If there genuinely would be serious adverse consequences from you coming in at a reasonable time and not being able to get a desk then I think you should raise that with your manager as it's really disadvantageous to anyone who has caring responsibilities or disabilities that make it impossible for them to come in at crack of dawn, and presumably costs the company money in productivity loss...?
-Also, I'm not sure it's right that childcare is 100% your responsibility to solve. Does your DH really have no option but to work very long hours everyday at the office, or is he going along with this as the prevailing office culture? Of course if he is the main earner you don't want to risk his income, but are there small changes he can make? Could he commit to leaving at a normal time 1 day a week to pick the kids up from nursery/after school club (he can always do emails/work from home in the evening if he really feels this is 'slacking')? What about working from home a day a week or a fortnight? I know some very senior/successful people who manage this and actually by setting an example to their teams and giving them 'permission' to at least take a tiny step off the long hours treadmill, everyone is happier, healthier and more productive as a result. Worth just challenging his thinking slightly on this I think! Obviously if he is a brain surgeon, police inspector or other work that really can't be paused or brought home then ignore!