Thanks for the additional information. It does help. I'm sure you probably are aware of this but just in case, a pillow between your legs can help alleviate back pain. I have to use one myself due to back pain. Additionally, a pillow placed in the area may also keep the dog from trying to sleep there.
I suspect the dogs feel this is the safest area for them. It is fairly easy to move a leg or an arm or roll your upper body and possible pin a small dog. If there is an open space in that area, the dog probably feels that is safer. You may not even be aware of moving around during the night but the dog would be.
The best solution would be to train them to lay down in a place and stay there. The safest place would likely be at the foot of the bed if there is enough room that they would not be kicked. I understand you can not afford obedience training so let me give you a site that is great at teaching you how to train your dog. Be sure and click on the link to the page on obedience at the bottom. and links on subsequent pages leading to detailed instructions.http://www.schutzhund-training.com/training_theory.html
Training works best if you train at least 30 minutes a day (two 15 minute sessions). I would start making your dog work via the Nothing in life is free program (NILF). It is outlined below. http://www.pets.ca/articles/article-dog_nilf.htm
Having them sleep next to you might not be the best idea just in case you do shift during the night. My personal recommendation would be to not allow them on the bed or furniture at all. Dogs that are allowed on furniture or beds start to believe that they are equal to the humans in the house or even higher since they have teeth and can bite. Your pups are still young and I would expect to see more problems develop if you do not get them trained now.
I am not saying that they can not come onto your lap for you to pet them or show them affection, just make them obey a command before allowing them up and once you are done petting them, make them get back down. You can provide them their own beds either on the floor or on a small platform. They even have heated beds available or you might put doggy sweaters on them. If you can not afford doggy clothes, often infant clothing fit chihuahuas quite well just by taking in a little underneath. I've made many outfits for my own chihuahua in this manner. This would help keep them warmer at night if they are not in bed with you.
So the options you have are to block the area at your crotch with a pillow, continually remove them from the area every time they try and go there, or train them in obedience so you can command them to sleep in a specific spot if you do not just have them sleep off the bed.
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Since there have been recalls on certain foods, please check the following site to be sure the food your animals eat is not affected. If it is affected, contact your vet as soon as possible. Have your dog seen if they have any symptoms.http://www.fda.gov/AnimalVeterinary/SafetyHealth/RecallsWithdrawals/