I think you must be feeling a lot of external pressure for a meal, an event after the communion, etc. I am not Catholic but I know it can be a huge deal to DO something, provide food, practically a reception, etc. -- in some places, in some churches. Ask yourself: Does your particular, local church --the other parents, the priests, the teachers -- just culturally tend to expect that families do all this extra celebrating with extended family? Is this maybe a case of "Well, EVERYONE whose kids have first Communion here takes their extended family out to a meal after the mass...it's just what people 'do' here...What do you mean, you're not inviting every single person in your family?!" and so forth. If YOU, personally, are low-key and not into inviting a crowd for every event, then you are maybe feeling spoken or unspoken pressure to put on a show, frankly. You had your plans and now you feel they've been hijacked.
Well, that boat has sailed, and others are now planning to come. At this point I would do as you've done and order as much of the food as possible (do not spend your weekend making things) and have, in your head, a firm "end time" for the gathering -- It can help to be able to say, "Sally has a (event) at 4:00 so we'll need to wrap this up at 3:00" or whatever (as long as it's true!).
But I'd really think through whether you and your husband, too, are caving to expectations along the lines of "That's what families usually do here" and the implied corollary "And if you don't do that, you're a bit strange...."
I don't think there was anything wrong with your original plan to have only the godparents and their kids there. Did you not originally invite your mom because you have some other issues with her, or because you knew that inviting mom equals inviting at least one if not several others too (your sisters)? I admit that not inviting a grandparent -- if she's close t your kids -- seems a bit odd to me, but it's possible your mom is a handful or you know that inviting her always means other issues crop up.
I know first communion is a huge deal. But it's a huge religious deal, and is also a social event only if you choose to add that onto it. It's a choice, not an obligation.
At our church, when kids were baptized last Sunday, the church had a beautiful cake to honor all the kids who participated and everyone in the church stayed after the service for snacks (provided by the church as they are every Sunday so that wasn't anything out of the ordinary) and the cutting of the cake in front of all the kids. Simple, easy, no pressure, and it acknowledged all the children who were baptized that day.