Ok, months not weeks, my mistake.
My point is - how you approach this with your employer is going to be slightly different. They've either changed your terms & conditions without discussing and agreeing with you. Or, you took a job with a variable rota which worked well for the first while, but isn't working so great now. Or you started on lesser hours than you knew it should be? Knowing which of those is the case, will help your argument about whether they should or could change your hours (back) again.
That then shapes the future - is this a temporary blip? Is this a rota which means a different shift-pattern? Is this what you signed up for? Because if its a blip, you can see it getting changed back. If it's different shifts each week, that allows you to sort childcare accordingly. If this is how it's supposed to be, then at least you know, and you either are happy with those arrangements or not.
FWIW, I think every job is difficult in the early stages, until you find your feet, know people and how things are done, and until you settle into a routine. Question is, whether you think this routine is the one you want.
As an aside, you may want to make sure you & employer have the same idea of "my kids come first". I'm sure most working parents would say the same - but equally your employer contracts you for a certain role at a certain time. Which goes back to my point - you both need to be agreed on what your hours are, and when / how they can change. If you want more time at home with your kids, that's absolutely fine; if you want to be home 2 hours earlier, fine - but is that flexi-time, is it back to part-time hours, is it working from home etc etc.
And go easy on yourself ..... Easier said than done I know, but a lot of our stresses in life we are guilty of creating, so can just as easy get rid off :-)