I was reading Crayon Shin-chan—a Japanese manga—and came across a frame where the characters' speech bubbles contain circles, triangles, and crosses where words should be. I asked a couple of my Japanese friends about what the shapes meant but none of them could tell me what they meant or why they were used.
Those are just placeholders. You should understand that instead of a meaningful text those symbols were inserted in order to avoid putting actual text. That can be done for various reasons: censor a word, make the reader unaware of something the characters are aware of.
That being said, here, judging from the image the characters seem like they are quarreling, thus we might think that the placeholders are just used to cover up inappropriate words (for the audience).
Those symbols do not mean anything. You, as a reader, can replace them by actual words if you so desire.
From the surrounding words, the double exclamation marks and the プンプンマーク placed over the heads of the Mom and Dad, the readers will know instantly that they are having a heated argument about their trip three years ago. The content of the argument however, would be irrelevant to the story. 「プンプン」 is an onomatopoeia for expressing anger.
The surrounding words that I am referring to are 「だいたいいつもおまえはな」("You know, you always..")、「なによあなただって・・」(Whatcha talkin' bout? It's you who...)、「そーゆーあなたこそ」、「だったくせに！！」, etc. I will not translate everything here for free.
Another big reason that the content of the argument was made unclear on purpose, IMHO, is to make Shin-chan's goofy remark stand out and make you laugh at the end.
"Hey, how old was I then? Was I as cute as I am now?"