I'm not sure how common it is specifically on beagles, but females are prone to mammary tumors. Having them spayed (especially at an early age), decreases this risk, which is why I asked if they were spayed or neutered.
I've had several of my seniors get lumps, including a couple of my dachshunds, my pointer mix and my shih tzu. For us, the vet did surgery, removed the lump, and sent it off to the lab for a biopsy to let us know if it was cancerous or not. Thankfully, none have ever came back as cancerous, and all were benign.
I suggest getting your guys to a vet as soon as possible, so that the lumps can be removed and properly biopsied.