when I was younger I would have the same problem.. I would be crunched on time but hit for an hour or two each day until late middle school.
What made my dad really keep my attention on tennis was that he always reminded me of how much potential I had. Tell them that if they work hard than the sky is the limit. That pushed me to become a really good player.
Keep in mind that once you get into highschool, a lot of times you can get out at 12:30 ... I am a junior and since freshman year I have had a 0 period and went to school from 6:45 - 12:30 and went to a tennis academy and could play for 3+ hours, leaving plenty of time to hang out and do homework.
Another way to make it not sound like a chore for your kids is instead of telling them.. "ok time to play tennis" ... ask them on the days you play with them "do you want to go play tennis" as if its up to them. after a year or two of saying "yes" to "do you want to play tennis" they will think that they are the reason they are playing which will develop a true love for the sport.