In Japan, they say that children finish doing everything as gratitudes for their parents by the age of three.

I don’t think so, because my son who is four and a half years old tells me good things and begets nicer day by day.

Becoming a mother is a really great thing.

I couldn’t imagine that a little baby loves me all the time no matter what until now.

My son often tells me that he loves me the mostmore than anything else in the world.

That is grammatically fine, but I think this sounds a little bit natural.

In Japan, they say that children finish doing everything as gratitudes forstop trying to please their parents by the age of three.

This is my best guess at what you were trying to say, my dictionary was not very helpful at translating 親孝行.

I don’t think so, because my son who is four and a half years old tells me good things and be niccomes kinder day by day.

We need to use "become" to translate なる. Nicer is fine but I think kinder fits a little better here.

I couldn’t have imagined that a little baby would loves me all the time, no matter what.

I feel that he gives me more than I give him.

全く上手で足りるお母さんだ絶対きっとでしょうよ！絆は美しそうですね。

In Japan, they say that children finish doingby the age of three children complete everything asout of gratitudes for their parents by the age of three.

I looked up 「親孝行おやこうこう」and it translated as "filial piety". An idea not common in Western cultrue(I wish it was...) which is likely why people had trouble correcting it.

I don’t think so, because my son who is four and a half years old tells me good* things and beis nicer to me day by day.

*"good" sounds a bit unatural, i think that if you used "nice" it would sound more natural. But it is up to you. ;)

I couldan’t imagine that a little baby loves me all the time no matter what.¶
or¶
I can't imagine that a little baby loves me so unconditionallly.

The second sentence is a common and more natural way to say essentially the same thing. It's used very often.
"no matter", unconditionally is kind of like saying "no matter what happens, without me having to do anything for them or in return they will always _______"
"Unconditional" is a word often used to descrbe the love between a mother or father and their child/children.

In Japan, they say that children finish doing everything asstop showing gratitudes for their parents by the age of three.

I'm not quite sure what you meant in this sentence >.>

I don’'t think sothat's true, because my son who is four and a half -years -old son tells me good things and begets nicer day byeach day.

Becoming a mother is a really great thing.

I couldn’'t have imagine thatd having a little baby that loves me all the time no matter what.

I don’t think so, because my son who is four and a half years old tells me good things and be nicer day by dayonly gets nicer as the days go on.

Becoming a mother is a really great thing.wonderful!¶

Absolutely correct! In this case you can use a different word like “wonderful” to make the phrase more effective