Apologies if I get anything mxed up UP, I have rtft but not all at once (busy day!)
Firstly, you CAN do this. I think it's totally normal to feel like this when returning from mat leave, and as teaching is a job where you have to hit the ground running it's quite stressful. I'm back in a couple of days after may leave for DC4 and I feel a bit like this. I'm comforted by the fact that I've now felt a bit like this twice before and I'm still standing! You can do this. After a week you'll be back n the swing of it. You'll make efficiencies where You need to and it'll be ok.
Equally, if it isn't ok, you CAN walk away. If your husband earns about ex what you do, you could stop working and reduce your childcare costs and it would be fine. You may have high outgoings at the moment, but there will be ways to manage. You can come back to teaching after a break -how easily depends on your exact position, but you could do it.
Is there anything you can do to make the evenings/nights easier? It's really hard to make changes when you are getting very little sleep. Lots of people in here can give advice. Even if you're not ready to make changes now, your children will be sleeping better in the future. 2 of mine have been dire sleepers in the toddler years, no trouble at all now. It will get easier one way or another, but perhaps we can help.you nudge it along a bit faster? I'm very strictly no tears at bedtime, I just can't stand crying when I need to be calm and getting relaxed myself.
Your DH's hours prevent him getting much involved in bedtimes etc. Can he pitch in more at the weekend? My DH has certain activities he does with the kids at weekend to help free up a regular slot for me of known length so I can plan on using it.
Presumably you are part time because you want time with the children (I'm also 3 days a week), so you wouldn't want an extra nursery day. However, you could ask aroumd child minders - they are often full for school run times, but may be below capacity in the middle of the day. Something like 2 hours twice a week for 3yo at a time when two tends to nap might buy you a bit of time.
If you give us more details of what might help, you might get more tailored advice.