half除了有“一半”的意思之外，还有“大约是一半的”的意思，即大约是半数。我觉得这句话就是说，他的这些孩子里，有差不多半数是男孩。下面是half的解释，或许能解答你的问题：
half
[hB:f]
n.
一半, 半时, (球赛)半场, 半学年
adj.
一半的, 不完全的, 半途的
adv.
一半地, 部分地, 相当的
half
half
AHD:[h²f, häf]
D.J.[h#f, h$8f]
K.K.[h#f, h$f]
n.（名词）
【复数】 halves[h²vz, hävz]
Abbr. hf.
缩写 hf.
One of two equal parts that together constitute a whole.
一半：一起组成一个整体的相等的两部分中的一个
One part approximately equal to the remaining part.
半部分：与余下的部分大致相当的部分
Informal A 50-cent piece.
【非正式用语】 一个50美分的硬币
Sports
【体育运动】
One of the two playing periods into which certain games are divided.
半场：某些体育比赛被划分成两个阶段，其中之一被称为半场
A halfback.
中卫
Chiefly British A school term; a semester.
【多用于英国】 半学年：学校术语；一学期
Half an hour:
半小时：
a half past one.
1点半
adj.（形容词）
Abbr. hf.
缩写 hf.
Being one of two equal parts.
一半的：两个相等物之一的
Being approximately a half.
大约一半的
Partial or incomplete:
部分的，不完全的：
gave me a half smile.
对我似笑非笑
adv.（副词）
To the extent of exactly or nearly 50 percent:
大约一半地：正好或差不多达到百分之五十的程度：
The tank is half empty.
油箱是半空的
Not completely or sufficiently; partly:
不彻底地或不充分地；部分地：
only half right.
不完全正确
by half
By a considerable extent.
以可观程度地
By an excessive amount:
过度地：
too clever by half.
过于聪明
by halves
In a reluctant manner; unenthusiastically.
不热情地：采取一种勉强的态度；不热情地
in half
Into halves.
成为相等的两半
not half
Not at all:
一点儿也不：
“Fancy housing? Not half likely, ma'am”(Russell Baker)
“上妓院?根本不可能，女士”(拉塞尔·巴克)
not the half of
Only a fraction or a small part of.
只有一个片断或一小部分
Middle English
中古英语
from Old English healf * see skel- 1
源自 古英语 healf *参见 skel- 1
The phrases a half, half of, and half a or half an are all correct, though they may differ slightly in meaning. For example, a half day is used when day has the special sense “a working day,” and the phrase then means “4 hours.” Half of a day and half a day are not restricted in this way and can mean either 4 or 12 hours.
短语a half，half of 和 half a 或者 half an 都是正确的， 尽管它们可能在意义上稍有不同。例如当day 当工作日来讲时， a half day 这个短语的意思是“四个小时”。 Half of a day 和 half a day 的意思不限于此， 它们可以表示四小时或者十二小时
half
[hB:f; (?@) hAf]
n.
halves
半数，一半
We had half each.
我们每人有一半。
Half of them are here.
他们有半数人来了。
Wait half a mo.
等一等。
Half a moment!
稍等片刻!
半币，半票
Give me a penny for 2 halfs.
给我两枚半便士的硬币。
半学年
（球赛等的）半场，半局
Neither side scored in the first half.
两队在前半场都没有得分。
〈足球〉中卫
by halves
不完全地；不彻底地
We do nothing by halves.
我们做什么都是彻底的。
go halves (in sth.)
与人平均分摊费用
half
adj.
半数的；二分之一量的
half a minute
半分钟
There are no half measures in scientific research.
科研工作是不能搞折衷的。
These apples are $2 a half dozen.
这种苹果两美元买六个。
half
adv.
部分地；一半地
He was half asleep.
他半睡着。
We were half convinced.
我们有点信服了。
半点钟
half 7, 8, 9, etc.
七点半，八点半，九点半等。
It is half past six.
现在是六点半。
by half
（前面与too和adj连用）过分
too clever by half
过分聪明
not half
极，非常
一点也不
not half long enough
根本就不够长
half
来自古英语 healf
half
whole
half
half
AHD:[h²f, häf]
D.J.[h#f, h$8f]
K.K.[h#f, h$f]
n.
pl. halves[h²vz, hävz]
Abbr. hf.
One of two equal parts that together constitute a whole.
One part approximately equal to the remaining part.
Informal A 50-cent piece.
Sports
One of the two playing periods into which certain games are divided.
A halfback.
Chiefly British A school term; a semester.
Half an hour:
a half past one.
adj.
Abbr. hf.
Being one of two equal parts.
Being approximately a half.
Partial or incomplete:
gave me a half smile.
adv.
To the extent of exactly or nearly 50 percent:
The tank is half empty.
Not completely or sufficiently; partly:
only half right.
by half
By a considerable extent.
By an excessive amount:
too clever by half.
by halves
In a reluctant manner; unenthusiastically.
in half
Into halves.
not half
Not at all:
“Fancy housing? Not half likely, ma'am”(Russell Baker)
not the half of
Only a fraction or a small part of.
Middle English
from Old English healf * see skel- 1
The phrases a half, half of, and half a or half an are all correct, though they may differ slightly in meaning. For example, a half day is used when day has the special sense “a working day,” and the phrase then means “4 hours.” Half of a day and half a day are not restricted in this way and can mean either 4 or 12 hours.
参考资料：金山词霸2005
本回答被提问者采纳