Cbt therapist here op. The worst thing you can do with intrusive thoughts is try push them away, it only makes you have them more. Like the pink elephant experiment, you can think of anything in the works, literally anything but not pink elephants....:most people are now thinking of pink elephants.
Remember thoughts aren't always true, just because your having it doesn't mean it's factual, your more likely having the thoughts as they make you anxious, you push them away so they pop back I , you then thinknits returned it must be true etc. ocd tends to present itself as the morale opposite of the person so the fact your concerned about your children tells me that your actually a very caring mum who would hate the thought of being a bad mum, this is why the thoughts are affecting you so much. We all have intrusive thoughts, but the intrusive thought that you need to buy milk on the way home doesn't hold as much emotional affect so we just brush that one off and forgot we even had it, being a bad mum on the other hand sticks, you remember that one as you hated having it.
I would aim to access some cbt via the nhs, cbt is the recommended treatment for ocd. You can also try an anti depressant, something like sertraline and there are lots of good self help books you can buy that are likely to help you at least understand ocd. Good luck op