ahem, well, XH is huge! (wouldn't tell him that thou)! and was worried about finding someone who would ahem 'make the grade' but you know what - it's not fair ont he bloke to compare the size of his to your x's - althou to be fair the bloke who I was with a wile back was well - found wanting - was small, (a lot), and it well, wasn't used in the right manner! current beau however isn't mahoosive, it's a nice size - but the key thing is he knows how to use it.
(am talking about never having the 'O' with XH unless it was manual prior to penetration - but with new beau it's well - everytime, and fantastic) ahem.
anyhow enjoy your new man, you've done the worst bit by getting undressed I reckon, and as long as he's concerned about pleasing you as well as his own pleasure, and like wise you are you should be fine.
IMO it doesn't matter that you've had two children, or that he has a slim penis. Sex with a new partner is always going to feel different than it did with your last and you just have to experiment a bit with different positions to find the one(s) that work for you both.