I have seen this Blocker family for the first time when I was running a daycare. They dropped their 4 toddlers at my daycare. I checked them with master controllerand found out that their household status is homeless and the parents are a singer and a magician. But then a week later they moved in one of my sims household and started to live together. There were already three member family in that house and the Blockers just joined them.

First of all, I don’t want this kind of big family in town. Is there anyway that I can control the number of the homeless family members? The other day, 7 member (parents and 5 children) just suddenly moved into my town and they were originally also homeless family as well.
Also, how can I stop this homeless family moving into my town? I’m ok with single, couple with one or two children but i don’t like these big families moving in...

I think you may have a combination of forces/mod settings in play here that most of us do not use as what you are describing is pretty unusual. I've never run an active daycare, so have to guess that if there aren't (enough) resident babies and toddlers, the game will generate some so that your active daycare provider will have something to do. That's one force.

But the homeless virtual "household" cannot have babies or toddlers in it. Therefore, the game itself or SP, not sure which will react first, will do its best to move them into some resident household, almost any household, as soon as it possibly can. That's a second, somewhat opposing force. The homeless "household" can have children though, that's where the newspaper delivers come from, but these children Service Sims typically do not need daycare as they take care of themselves.

The third one, if you are using NRaas SP for story progression, is SP's Manage the Homeless. If that setting is left on, the mod will try to find suitable housing for some homeless sims in some circumstances. This might be one of them. I would suggest switching that setting off.
NRaas > SP > General Options > Options:Households > Manage Homeless

Finally, we can control the maximum size of households that SP Immigration creates and moves into empty housing. But what you are describing doesn't sound like SP Immigration to me, it sounds like some other factor emergency-moving existing homeless sims into existing households, not granting them official households of their own.
NRaas > SP > General Options > Options:Lots > Options:Immigration/Emigration > Whole Family Immigration Size > (default is 8)
(Immigration Gauge must be >0 to see or adjust this option)