At our baby (admittedly not "toddler", but can't see why it won't work the same) swimming you start by saying their name, then "ready" then "Swim!" and pour a cupped hand full of water over their face, wiping it off with same hand as you pass over the face.
You do this on Week 1, and practice every night in the bath.
To promote kicking in the water, child is belly down in the water; you support with hands on chest/ underarm, thumbs at front of shoulder joint and pull the child along in the water, pulling towards you and then walking away. Keep child's head above water, your shoulders in the water.
Child upright in the water, held under arms and "swayed" from side to side in the water also promotes kicking.
Week 2, you can try a "facial" in the water. Name, Ready, Swim! and dip face in the water very briefly whilst holding as described above.
You only do "under waters" about 3-4 times each session, less if child is distressed. Keep practising with cupped hand of water in the bath.
By week 4 you should be able to get full body under the water, week 5 you can do a short "release" from one person to another under water.
If the child isn't "getting" taking a breath before going under, blowing into the mouth with cause them to take a breath before you dip them.
My baby has done about 8 sessions now and at 7 mths old, now closes her eyes and holds her breath when I say, "Swim!".