Grass cutting

So yesterday was the first time I cut the grass this year.  Last time I cut it, my children were six months younger.  At the time, Malachi took two naps, in the morning and afternoon, for two hours.  Alyssa could and would still play in the playpen.  Normally, I would put Malachi down for a nap and then put Alyssa in the playpen with toys.  She was fine, never tried to escape and I got the grass cut.  Yes, I got weird looks.  O well!  So yesterday I tried my tatics to keep them entertained while I was cutting the grass.  Malachi doesn't take two naps anymore so I had to take both of them outside with me.  The kids both love being outside so I thought that I could bring the playpen outside and put Malachi in it with some toys for him to play with.  I let Alyssa play in the garage and driveway.  She listens pretty well so I wasn't afride she would run out in the street or anything.  I also showed her where she could go in the driveway and kept an eye on her.  For the first half hour, everything was fine.  Next thing I know, Malachi somehow got out of the playpen and was riding on a little bike in the driveway by Alyssa.  I totally freaked out because he could have been hurt, or run into the street (He is at the running and not listening very well stage).  I didn't know he could get out and have never seen him try to do it when he was put in the playpen in the house.  He must have really wanted to ride the bike.  Needless to say, I only got half of the yard done.  I guess I will have to finish the yard during the next nice day, which lately has been few and far between.  Any advice for entertaining children (and keeping them safe) while cutting the grass?  Alyssa is fine, it's Malachi that I am worried about.  I guess my only option right now is to cut it while they are napping, or try to cut it on an evening when Leonard is home and it isn't raining or the yard isn't wet.  That has yet to happen.