I don't think your DH is the concern here to be fair, it's a bit of a red herring, the mechanics of that part seems to work perfectly fine, I am in agreement with a lot of posters about the children having to have long days because of it though, it seems quite excessive, and I can understand why your MIL is concerned about it, calling you lazy is just showing her ignorance though to be fair, if she wants to raise legitimate concerns she needs to learn to be constructive.
The part I honestly can't comprehend though is that you plan to volunteer evenings and weekends in the hope of potentially getting a job there? Which will take you away from the children even more! It's bonkers OP, you already volunteer there, so if there was a job going you would hear about it, besides if there IS a job going, you may not even get it (and you definitely won't if you are not even qualified enough to apply for it at the moment).. that idea seems quite selfish IMO.
I'm trying to be understanding but it seems that you are focusing so much on you and what you need, you don't seem to be considering the wider impact it may be having on your children at all, it's all well and good that you are happy, and they don't mind having really long days, but aside from the financial implications, everyone having to do more so you can choose to do less will start to take it's toll, from the outside you seem to manage perfectly fine when it is something you want to do, but you need to start learning to manage when it is something the children want to do too, by putting whatever strategies you need in place, your DH seems incredibly supportive which is great, but you can't just opt out of parenting.