Djeca su došla blizu bunara, ali im je mama rekla da idu dalje.

Because djeca is a special plural noun meaning children. It comes from singular dijete (child) and takes plural meaning, but in some agreement patterns it behaves like neuter plural.

So in the past tense:

• došla = the form used here with djeca
• not došli, which would be used with a masculine plural group

This is one of those forms learners usually just have to get used to:

• Dijete je došlo = The child came.
• Djeca su došla = The children came.

A confusing detail: došla looks the same as the feminine singular form, but here it is actually the correct form for djeca.

Su is the 3rd person plural present form of biti (to be) and it helps form the past tense.

Croatian past tense is built like this:

• present of biti
  • past participle

So:

• su došla = came / have come

Other examples:

• On je došao = He came.
• Oni su došli = They came.
• Djeca su došla = The children came.

Bunara is genitive singular of bunar (well).

The word blizu normally goes with the genitive:

• blizu kuće = near the house
• blizu škole = near the school
• blizu bunara = near the well

So:

• bunar = nominative
• bunara = genitive

Here blizu means near / close to.

So došla blizu bunara means:

• came near the well
• got close to the well

It suggests movement toward a place, but not necessarily going all the way into it or onto it.

Im means to them.

It is the unstressed dative plural pronoun:

• im = to them

In this sentence:

• mama je rekla djeci = mother told the children
• mama im je rekla = mother told them

So im is the indirect object.

Because the verb reći here needs an indirect object meaning to whom something was said.

• im = to them, unstressed dative clitic
• njima = to them, stressed/full form
• njih = them, but not the right form here

So:

• Mama im je rekla... = Mother told them...
• Mama je rekla njima... = Mother told them...
  This is possible, but it is more emphatic.

In neutral sentences, Croatian usually prefers the short clitic form im.

Because im and je are clitics: short, unstressed words that usually appear very early in the clause.

So Croatian often puts them near the beginning:

• ali im je mama rekla

This is very natural Croatian word order.

You may also see:

• ali mama im je rekla

That is also possible, but the sentence you were given follows the common pattern of placing clitics early.

Because the subject of that part of the sentence is mama (mother), which is feminine singular.

So the past participle agrees with mama:

• mama je rekla = mother said/told

Compare:

• tata je rekao = father said/told
• mama je rekla = mother said/told

After verbs like reći (to say/tell), Croatian very often uses da + present tense.

So:

• rekla da idu dalje = told them to go farther / said that they should go farther

This is a very common Croatian structure.

English often uses an infinitive:

• She told them to go farther.

Croatian normally does not say:

• rekla im ići dalje
  That would sound wrong here.

Idu is the 3rd person plural present of ići (to go).

It is plural because it refers back to the children:

• da idu = that they go / that they should go

So the logic is:

• mama rekla = mother told
• da idu = that they should go

Dalje means farther / further / farther away.

Here it means the mother wanted them to move away from the well, not stay so close to it.

So:

• idu dalje = go farther / go further away

In other contexts, dalje can also mean on, further, any more, but here the idea is clearly farther away.

Because ali means but and joins two clauses:

• Djeca su došla blizu bunara
• ali im je mama rekla da idu dalje

Croatian normally puts a comma before ali, just as English often does before but when joining two full clauses.