No level drain is involved though.
Red and blue slaadi reproduced by infecting living hosts. The red did so by implanting eggs beneath their victim's skin which grew into a blue slaad tadpole that ate the host from within. The blue infected the host with a lycanthropy-like disease called chaos phage that slowly transformed them into a red slaad.
What are some of the details that you were thinking about? What size is the frog? Is the frog able to polymorph or shape-change into a humanoid form? These kind of details.
You can avoid some really unpleasant connotations by making the frog spawn things from its back like Gizmo from Gremlins when they leave to ‘Tryst.’I’m not entirely sure how to build this monster, however this gist is this: the frog is entirely female and can ONLY reproduce with humanoid males. It starts with planting a big wet kiss that charms the victim and they then go Of for a tyst.
The frog is unable to polymorph & is roughly the size of a human, it’s charm effect is strong enough to effect dwarfs if needed though they tend toward non-magic resistant creatures. They are prime material plane beasts naturally & defend there young VIOLENTLY if needed!