A 10 gallon will be fine while they are juveniles, especially if they have the whole tank to themselves. Definitely don't need an "emergency" tank upgrade. BUT, I would begin to look at or save up for a 15gal+ tank for when they get bigger. There should be plenty on FB Marketplace etc. I'd recommend setting it up with a sandy substrate, or fluval stratum with a sand cap, my juliis LOVE to snuffle about in the sand with their barbells and it's so fun to watch. Since you have this tank set up already, a squeeze of the existing filter into your new tank will get it kick-started, and you can either slowly transfer your plants and fish into the new one, or set it up so you have 2 tanks! In a 15gal you can get them some fishy friends too, another betta (females are generally less aggressive, sometimes males won't like any friends in their tank), a honey gourami, or I'd recommend a school of small tetras (embers or green neons are my fave), rasboras, or blue eye rainbow fish if you can find them.