Demonstrate what it means to flex and to point their feet, then move their feet for them from flexed to pointed and back again, so that they really feel the ankle and can isolate that feeling.
If your child is enrolled in class levels 1-3 have your child sit on the floor and lean back on their elbows. If they are in levels 4, 5, or 6 have your child sit on the edge of the chair and brace themselves with their hands. Leaning their torso back slightly for balance have them kick from the hip, keeping their legs straight, their ankles loose, and the kick small and narrow with toes pointed. Try timing them. Start with short amounts of time like 15 seconds, rest, and repeat as many times as they are willing and able to do it well. Then slowly build the duration each day.
While doing a round of kicking touch your child’s upper thigh to show them what muscles are used to produce the kick. This exercise doesn’t give them a feel for how important the ankles are, because air resistance is so much less than water resistance, but they will get a good feel for keeping their legs straight and for the range of motion from the hip. Have your child start kicking slowly and then without compromising quality have them pick up the pace. You can count out loud: “1, 2, 3, 4, 5, and 6” then repeat, and have your child keep your rhythm. Keep checking…How straight are their legs? Are their ankles flexible? Are they flipping their feet like they are trying to flick off their shoes? The level 4, 5, and 6 kids will also start to build core strength because they have to balance on the chair while kicking. (Core strength is also essential to swimming properly.) Kicking is a critical swimming skill yet many new swimmers kick freestyle as if they are riding a bike. It can take a long time to break that habit. If you can spend even just 10 minutes a day doing kicking drills with your child it will help build that muscle memory and translate to progress in the pool.