Lots of options, but will the bike be used in hilly areas? If so, a mid-drive is the best option IMO. If not, look at Swytch (I don't like handlebar bags (in this case fro batteries). Otherwise Luna has an attachment system that would permit the battery to be installed in many places on the bike or purchase a rear rack and mount the battery there. The photo shows where I mounted a battery on my daughter's bike and works well for her.
