Hmmm I am panicking too and I only have 2 children. It's not easy. DC1 is nearly 6, DC2 is 18 months. Such a big difference in their needs and what they are physically able to do and enjoy.
I find it extremely hard to keep them both happy when DH isn't here, I feel spread very thin so I absolutely take my hat off to you for coping with more!
I actually had a bit of a melt down just before summer holidays last year. The thought of spending weeks in the house with a then 4 yr old and a very difficult 7 month old was horrific and I ended up having counselling! (I suffered from PND and still do feel the anxiety return sometimes).
I am really not looking forward to it this year either. Memories of last year are flooding back as we edge ever closer!
Anyway, I am basically saying that I feel your pain! My confidence is at an all time low. I just feel like I am a crap snappy mum and I have zero energy.
Getting out the house for me is essential although it has to be timed around DS2's nap at midday. I am planning on getting my good friend round to help entertain them both 
she's also suffering from depression, loves my kids and wants to get out her house more so we've made a pact to help each other out. Is there any support you have locally? friends, family??
I am planning on taking them swimming once a week with my friend. We are going to the local arts and craft centre once a week too. My friend will come to sit with DS1 (she's actually an artist and is very good with craft etc so she'll enjoy it too!) and I can walk around with DS2 in the pushchair.
Other than that I am planning on getting the paddling pool out a lot (weather permitting) and I fear the Kids TV channels will be abused :(
I wish I was better at this. I wish my boys could play nicely together and do arts and craft etc together but with their ages at the moment it's just pure stress. DS2 is not yet walking (adding to his whinginess and frustration) so he's so limited as to where we can go and what we can do. He's currently having physio weekly and having support boots made for his ankles so hopefully that will bring him on a bit and by next summer holiday life may be a little less stressful. (Not holding my breath though!)
How old are your children? I find anything under 4 is very tricky. They don't really know how to play together, toddlers are tempreamental and make getting out the house a complete pain in the arse. I think lowering standards on housework, even lowering standards on how much 'entertaining' them both. So what if the TV is on, so what if you aren't baking fairy cakes with them every day? Do what you can to survive! I am trying to not beat myself up when DS1 wants to sit and watch TV or play on his tablet. I feel guilty but I know it reduces the stress and pressure on us all. I am determined to get through it.
Sorry I'm not much help. I am dreading it although strangely looking forward to having DS1 at home, I feel like I'll just end up snapping and becomming shouty and stressed and that's why I'm not having any more kids! Hopefully your older children, depending on their ages can entertain each other in the garden or at the park etc. Good luck. x