(1) It seems to me that the difference between the 2 Gemaras is that in Gitin the father left his family entirely and went out of the country. Rashi 6b DH va'Yitnu explains that he left the wife as an Agunah - a deserted wife - and as a result they neglected the Mitzvah of being fruitful and multiplying. Rashi adds that they were making light of giving birth to boys and girls. So the problem in Gitin was not so much that they were not treating properly the children they possessed, but on the contrary they were not making a sufficient effort to bear children.

(2) On the other hand, the Gemara in Eruvin refers to someone who stays at home and does not run away. I suggest that the Gemara in Eruvin is teaching us that one should not pamper one's children, and by not giving them everything they want; this in the long term turns out to be for their own good. The expenses saved by this way of life will also help the father to devote himself to studying Torah, without having to worry overmuch about making a living. It could be that Eruvin does not mean that a person should actually be cruel to one's children by denying them their genuine needs, but rather that the main thing is that the children should not recieve more than they require.

(3) I would explain this a little further by pointing out that sometimes a child begs his father for a candy or a special toy. This can be quite a difficult trial for a parent. The parent realises logically that the present the child is asking for is not crucial but on the other hand he has a guilty conscience if he does not yield to the child. It is a sort of cruelty to the child not to indulge him, but it is something which has to be done.

(4) I am reminded of something that the Chazon Ish is reported as saying. He said that to run a "Gemach", a free-loan fund, one needs to possess a certain amount of cruelty. Obviously it is not really cruel to demand that the debtor pays back, because he took the loan on the understanding that he was going to return it. However, practically speaking, anyone who has experience with these matters knows that it can be quite a painful experience actually to make people pay up. The Gemara in Eruvin may also be referring to a similar sort of valid and permitted "cruelty".

(5) According also to Tosfos here DH va'Yitnu one sees that the problem is that the husband leaves the country and does not support his family. If he stayed at home but merely insisted that his family not lead a luxurious way of life, Tosfos would agree that this is acceptable behavior.

(See also Teshuvas Chasam Sofer CM #9 DH Akati. However, see Tosfos Kidushin 29b DH Ha who writes that the people in our Gemara were not doing the right thing, which does not appear to be consistent with the Chasam Sofer).

Shavua Tov

Dovid Bloom

P.S. Wish me Mazel Tov, because my son just got engaged!