If you only get a few seconds it's best to go for the back ones if you can because they have to last longer in the mouth and also have more nooks and crannies to get food stuck in.
TBH I think it's pretty hard to brush little people's teeth so I'm just trying to establish the habit in the hopes of perfecting technique later.
Apples do a pretty job of cleaning the surfaces and massaging the gums and cheese is quite good for neutralising plaque acid (full of calcium too) so if you can get them to eat them it's good for their teeth.
Yasin won't let me touch his teeth so I'm really grateful that he gets a second set one day.