Hey Newbie,
As you know, my little ones are exactly the same age as yours, and currently they do a combination of arrangements which they really enjoy but it's tricky sometimes staying on top of what they're doing/when.
At the mo, they both do two full days at nursery every week which they really enjoy. This frees me up from around 8am when hubby leaves the house with them, until around 5ish when they all come trooping through the door. One day a week my 3 year old goes to the local playschool which is 9-3, which she loves, and my lovely Mum has my youngest at my house, then goes to collect the older one and has them both until around 5ish. This should mean I can work from 9-5, but it doesn't work out like that. Now, as my work has gone loopy, my 3 year old goes to playschool on the fourth morning while my mum has my youngest, then they go together with my aunt and uncle for the afternoon. This gives me four full(ish) days to work, plus evenings and a bit at weekends, and gives the girls two full days at nursery, time with family who love them and they love, plus a day at playgroup for my older child.
It all works really well and I feel less guilty than if they were going to nursery for four full days, as I know the girls adore seeing their grandma and other relatives, but the days where they're at nursery are usually my most productive as they're not coming/going/needing taking/picking up or trotting down the hall to come and say hi to Mummy. Also, and it's a huge also, it's cheaper. We currently pay a staggering £750ish for two days at nursery/a day at playschool (my 3 year old now has her funding for 15hrs which barely touches the sides) and if we were looking at four full days (approx £1300ish) that would put a dent my margins.
Currently, I have more than enough work to fill full-time hours but I'm resisting taking that step as I want to have one day devoted to the girls and then the weekend, and so for now I work in the evenings while they're asleep, and my eldest and hubby are watching tv or talking about boy stuff. It's not ideal, it feels like I'm burning the candle at every end possible, but it's working for the kids, it's just me that's feeling a little too stretched and in need of some down time. But hey, that'll come. When they're 19.