IMO, I think you probably should have submitted the bill to your medical insurance. You can always submit charges to your insurances, we've just gotten very used to working with providers who do the "automatic claim forwarding" so we don't ever have to worry about it.
The problem with them submitting it to your insurance is medical insurances are super-sensitive to the coding of claims. If they don't code it correctly, then they'll get it back telling them to resubmit.
Insurance really isn't as simple as a lot of us seem to think. My dad was an optometrist (not quite the same, I know). If someone had told me that their insurance would pay for their visit even if my dad wasn't an in-network provider for them, I wouldn't have had any idea of how to start the process to be paid.
For those who say "it's her job" - it's really not. It's a service - a convenience medical/dental/optical offices provide for their clients. It's not a requirement and it's complicated. Yes, we've all come to expect not to have to handle submitting claims to our insurance, but someone still has to. Again, if it was a small office and they only dealt with a select handful of insurers, I'm surprised she didn't simply say they don't do it and she wouldn't.
In a small dental office, they may not deal with a lot of different insurances. And she did say they don't deal with medical insurance. Some do, some will not.
I would probably call your medical insurance and ask them what the best course of action would be.
I do think the receptionist was really rude - she could have given you additional options.
BTW - have you been paying for dental insurance for your little one? If so, I'd request a refund!