Hi Proverbs.
I was in your shoes a few years ago, although I did it as a second career lawyer in my thirties.
You will be surrounded by trainees arriving very early and staying late, don't worry. Law is about billable hours, paid invoices and results, just being at your desk doesn't make you a good lawyer.
On the job front: I would make every single minute I was in the office productive, no chatting, no faffing. To do list written every day before leaving so that something concrete achieved within minutes of arriving in the morning. Coffee drunk before arriving so I wasn't delayed by making a drink.
I always took home tasks that could be done at home, research, drafting, writing up minutes, to counter the idea that I was a 9 to 5 er.
On the home front: when I was at home I was at home. Didn't look at work emails, or the Gazette, or legal websites. I always changed out of work clothes and jewellery the minute I got in, spent 10 mins alone in the bedroom just relaxing then into home clothes and being Mum/Wife. Work brought home was done after kids were in bed, I would set a timer for how long I was prepared to work, do it and then stop.
It is bloody hard being a trainee and even harder doing it as a parent of young children but I believe that in the long run it makes you a more resilient lawyer. If you've done a good days work after being up all night with a teething baby you won't be daunted by very long contract negotiations or nights on call for criminal work. Also the discipline of putting your children's needs first should stop the anxiety that can overwhelm many lawyers, family life can put the work into perspective.
It's incredibly hard work being a parent, you know this already. Being a parent when you are training is super tough and training in an environment as relentless and competitive as law can seem impossible. I can't speak for anyone but myself but I genuinely believe my training and career make me a better parent. I concentrated on being fully 'present' with my children and never felt guilty about not thinking about them when I was at work. They both say that knowing that I was studying and training when I was away from them inspired them and helped them realise that jobs/careers/possessions have to be worked for, adults don't just acquire them by right!
In practical terms I used my income to buy in services that gave me more time at home; cleaner, online supermarket shopping. I thought of it as an investment in myself as a parent, I would rather spend an hour in the park than an hour in the supermarket.
Above all don't let anyone make you feel guilty about training when you have young children, lots of women don't have the luxury. Your training contract is a huge achievement and it's an investment in your children's future as much as yours.
Good Luck!