Hey so.... firstly there are a tonne of response on here that tell you to run. My answer wont tell you to run because otherwise I would have followed this advice. My answer is, do what is right but be very prepared!!! It is SO much harder than it looks on the surface, I am 5 years in and just about surviving the rollercoaster.
I was in an incredibly similar position. I was 29 (freshly divorced myself - no children) and my OH was divorced - amicably - 46 with two children. He was my boss.
Work - Now he is not my boss, we are joint directors in his own company and I am employed elsewhere. Whilst it was great working together as you get all this extra time - having something for you is really important.
His children and Ex - What others are saying is 100% right, you will look after his children, their lives will totally dominate yours and if like me you think - well they will be adults soon - this literally makes no difference! If you are serious about him, you have to put his children first, he will not appreciate feeling like he has to choose between you and his children and his ex will be wary of someone not prepared to do this. In my case this meant, decorating bedrooms for each of child which felt like giving up part of ''my home'' to a sudden busy teenage comings and goings, continuously changing plans at weekends to have them stay/feed them/drive them everywhere, dealing with teenage issues and family fall outs, always cooking. What I mean is your life will never be the same and as a young person this will be draining.
Leading on from this if your friends are around your age they will really struggle to understand and be there for you, you will be so busy all of a sudden with 3 ready made children you will struggle to hold onto these friendships.
Money - This is a tough one. You will end up spending money on his children, either out and about, paying for a house that is big enough for them to stay in. If you are serious about him then this is totally fine. In my case, my partner was paying 100% of the bills in his old house and I was paying 100% bills in my house. My ability to do this funded my partners ex and childrens life for about 4 years until they sold the family home. Don't get me wrong it tore me up and I was totally skint every month, but if I want to spend my life with a person and he has children they are part of my life and responsibility too. It might not be this extreme for you depending on financial circumstances but at some point you will be out of pocket for his children and you need to be ok with this.
His ex- This might sound really harsh, but you will feel like the 'other' women even though he is divorced. He has children, his ex has the children with him. They will spend alot of time communicating, you will not know what is going on all the time, you will be the last to know about plans and it will feel like she has some level of calling the shots in your life.
In my case, my partner had a friendly relationship - this is obviously great for everyone (apart from me!) because this meant dealing with them often doing things as a family which I found tricky. Birthday meals out, schools results treats, a graduation from uni. All while I sat on the sideline feeling like a third wheel. BUT again I love him and his children and it wasnt about my feelings in those moments it was about making sure their kids has the love of both parents and showed a family front.
Anyway. All I am saying is you have to do what is right for you. But this will NOT be easy and it will make you feel a little out of control of your own life by entering someone else's family. You will have to be so grown up all the time, you will be jointly responsible for someone else's children and that terrifying. Good luck! I wish you all the best and you deserve to be happy xxx