Can a married person who does not have children ordain if he has the consent of his parents? Does his wife has to give permission? How much contact should he keep with his wife after he ordains?
A married man need not ask his wife for permission. Even the permission from one's parents was once disregarded by Sariputta (in ordaining Revata).
According to the rules (Pāc. 80), though, a woman has to get permission from her husband. This seems to be a carry-over from patriarchal Indian society. I would suggest that in societies where woman are seen as equals to their husbands rather than property, this rule should be broken. I know this isn't the direction of your question, but just for the sake of completeness, the point of the rule was that angry husbands posed a threat to the sangha (I was almost beaten in modern day Sri Lanka by one such jealous husband for ordaining his wife).
Assuming you are confining your question to men wanting to become bhikkhus, there is no requirement for consent from one's wife. Examples of married men becoming monks (ostensibly without the permission of their wives) are Ratthapāla, Culakāla and Mahākāla, and Yasa.
Note that having children is not a barrier to ordination (for women either).
As for contact between spouses after ordaining, the answer is pretty obvious; the spouse should be treated as any other lay person, though special care should be taken not to fall back into attachment with them. The story of Maha and Cula Kala is instructive in this regard.
Married people can ordain. Ideally you should get permission from your wife also though not explicitly required.
You have to limit the relationship to that of any other woman. I.e,
- no physical contact or being alone with her
- no flirting or anything such talk
- you should not get involved in any of her worldly problem like providing for her though you can make the arrangements beforehand
Ideally you should break ties of being your wife or special or significant other but still can be friends treating her like a friend or sister.
If your parents live you have to get their permission.
See the following for more information on the process: http://www.buddhanet.net/e-learning/buddhistworld/ordination1.htm