I like using the heaters you stick right into the tank, rather than have them clip on the side and hang in - there are a few cheap models that instead of being adjustable, they automatically heat to a specific temp (will be on the box) this is also helpful in preventing heater malfunction or inaccurate setting and frying the frogs.
I use this one in all of my smaller frog tanks - it is the Tetra Submersible Aquarium Heater
here is a link from petco's site so you can read about it. There are some mixed reviews, especially when it comes to bigger tanks, but I have used the 2-10 gallon size for all of my 10 gallons during the winter and have had no problems with it, even in my tadpole and froglet tanks.
Tetra Submersible heater
another brand you can look into is the Marina Submersible Mini Aquarium Heaters - I have not used these personally but they also seem to have good reviews.
also, for a 5 gallon, I assume you are getting african DWARF frogs, correct? a 5 gallon is acceptable for a pair (2 frog)
please make sure you know how to clearly identify the difference between an african DWARF and an african CLAWED frog - their needs are vastly different when it comes to tanks and they cannot be kept together.
pet stores will often label claweds as dwarf to increase the chance of a sale.
dwarfs - - have webbing on all four feet between the toes, eyes are flush against the side of their face, and are NEVER albino or the dyed colors.
claweds - have webbing only on their hind feet between the toes, eyes are ontop of their head and can come in a variety of colors
also, what are you planning on feeding them and how are you setting up the tank?
if you can, check out the link in my signature "Acf vs Adf - - learn the difference for the frogs"
there is a whole section dedicated to ADF and how to set up their tank and what their needs are