OP - If DP can move in with you in a way that isn't reducing the quality of life for your DCs, then do that, but be quite clear, you working fulltime rather than part time in order to make up the short fall in your benefits being reduced means that financially your DCs lives will be exactly the same, but have less time with their mum, this is reducing the quality of their life overall.
If you working full time meant that you would have more money so they could have more material things/better family security, then that might off-set the downside of them getting less 'mum time', but this isn't the case, financially there will be no change for them, the only upside to you working full time will be that you get your boyfriend to move in, that's an upside to you and him, not to your DCs. They will just see the quality of their life reduce in order to improve your boyfriend's.
You have to decide, who's quality of life is the priority, you DCs or your DPs?
Now, if he can find a way to move in that doesn't reduce your DC's quality of life (as in, he makes up the shortfall in the benefits you receive and pays the difference in the costs), then that might work out, but he doesn't seem to want too.
Oh, and my mum's best friend 'dated' her 'boyfriend' for over 10 years before he moved in with her, they both had DCs who were different ages with different needs and it wasn't possible to blend the two families without massive compromises, they took the very grown up decision to just date and not live together until they were at a stage when his DCs had grown up and left home and her DCs had reached teenage years. Not ideal for my mum's friend or her DP, but best for their DCs. They are still together another 20 years later, so waiting didn't stop them being long term commitment. If your DP loves you, he'll accept that living together isn't practical until your youngest has got to 7, or unless he hands over the bulk of his income.