I brush/comb Max's "undercarriage" a number of ways:
1. Laying on his back on my legs -- if your babies can't go flat on their backs, try sitting with your legs bent so that the dog is somewhat upright
2. Laying on his side in his bed, one side at a time, picking up his legs to get into the armpits or groin
3. Standing and sitting on the counter; crouch down and pull the hair out from underneath; when he sits, I can get most of his chest and stomach
Keeping the dog still: I worked with Max from the time he was 13 weeks old; I try to comb when he is tired and mellow or right before his walk -- he knows that he can't go out until he is presentable

; I praise him endlessly in a sweet voice or act silly to make a game out of grooming; I also give kibble or lamb roll rewards.
Max would rather play with toys, but he has come to accept grooming as another form of attention.
Hang in there!