Sounds great. They are supposed to be really neat little companions. I personally have never had one but I think they are darling (frogs). Just remember though that the main thing about putting a betta in a tank with ANYTHING is that it usually works much better to put the betta in the tank the last thing. That way the territorial little fishie thinks he is going into someone else's home and has a better chance of behaving himself. If you put the frogs in last he will see them as invaders into his territory and fight with them, more than likely unless you get a VERY docile betta. (and the chance of that is very slim) There is still no guarantee that there won't be any problems because, if you know how bettas are raised, they are not used to having anything in with them but patience is the key. They can learn sometimes with patience and some mild (not loud) knocks on the aquarium (taps with a finger or such - remember the sound in the water is magnified so don't scare the fish to death) to redirect his attention when he is misbehaving. Some quality time with just the two of you (with the betta) every day at least once is a necessity if you are going to have a happy fish. Talk to him for a while, handfeed him one pellet at a time (make him chew each one up so he doesn't just gulp them down and get a tummyache), sing to him, get a tall plant for him to sleep in (silk is better than plastic - less sharp edges to tear fins - real is the best) (they have to go to the surface to breathe so it keeps them near the surface at night - it is a great love gift for a sleepy little buddy so he doesn't have to swim so far at night)
I know you didn't ask for all that much information, but you see I love my Bettas all 4 of them at the present. They rule the roost at this house in a big way and are my constant companions and are my favorite topic. So when I get started I just do not know when to stop.....
Welcome to the Betta Board and to Fishlore.com. I hope I have been able to help you. We have a lot of fun here and take our fish seriously but love them to distraction also. Enjoy your tank and if we can help be sure to ask questions or explore the forum as you wish.
I think it will probably work too, and I second Rose's advice about which to put in first. One little warning is that bettas can be very competitive about food if they have other creatures in with them. Dwarf frogs sometimes take a bit of time to find their food--most frogs seem to like food to com to them rather than to have to seek out food. My betta will greedily attack bottom sinking pellets for instance before the catfish even get to it. You might need a way to distract your betta while you are feeding your frogs.
Also, I've noticed that while most fish that are not snail predators will ignore snails that even non-predatory fish, even gentle fish like platties will occasionally pick on dwarf frogs. I would still consider trying it but just have a plan in case they don't enjoy sharing together.
I have heard it both ways - that bettas and frogs won't get along (coming out worse for the frogs) and that they get along great. I would say... definitely put the betta in last, and observe them regularly to be certain that the frogs aren't getting beaten up. If they are, one option that would not require a whole new tank is to devide the tank in half. I use plastic canvas because the water can still flow through the holes, but not bettas. Use some aquarium caulking to seal the divider in there, or you'll end up with a betta or a froggy getting through. Trust me, I learned the hard way (fortunately, nobody was hurt except me when I had a heart attack at the realization that there were TWO fish in ONE compartment).
Make sure to put a lid on the tank . I don't know about frogs, but bettas JUMP.
Little side note, I wouldn't divide it if it is only 5g--that leaves hardly any space for the Betta of the frogs to swim. if for some reason it didn't work out you'd have to get a new tank or else take back the frogs or the betta. But I don't see why it shouldn't work.