When I had my first child at 19 me and my now DH worked full time, and ran our own house I was a little bit similar to you in the sense I didn't know what I would get benefit wise.
We did get some child tax credit? Which I'm sure has now changed to universal credit? And of course we got child benefit.
It might not seem like you're eligible for much if anything now, although we did seem to earn a little more than yourselves so I assume you should. However, once you have the baby you should fall into the criteria for other benefit and support eg housing benefit. If you earn under the threshold and are in receipt of the child tax credits it may also make you eligible for that? However, this was a while ago and it's all changed since and I've no idea how the system works now.
Also, there is nothing worse than a mother feeling like "she gets it all on a plate" where she is simply just looking for some financial help, as are yourselves. Maybe in reality some people do abuse the system but I can hand on heart say I'm a hell of a lot better off earning my own money than "claiming benefits" that said, I used it when I needed it.
The same for the sure start maternity grant, it's there to support mums that are financially struggling to help with one of costs for things they might need. A family who are working, earning there own money do not need to take a grant intended to help the struggling, surely that's just a bit greedy?
Good luck op, I'm sure you'll have a better idea now using all of those calculators!