If you are OK with your wee boy sitting 5 seats away from you (and he is OK with it,) then book the tickets. But it has to be ok with him, because the people may not want to move. As someone said earlier it's not that far away, 5 seats, but if you're not sure if your boy will be ok 5 seats away, then don't risk it. Many people would say yes they will move, but they are entitled to say no.
Also, as a few people have said, it's quite likely that the people in the row (one of the 5) will move. But as I said, don't assume they will.
Me personally, I have moved seats before in situations like this, and done similar things; like I have got up and stood up for the last 30 minutes of a train journey to enable a child to sit next to mum, (coz I'm soft,) and I let people in front of me in queues quite often if they have a lot less items than me, and I've done things like this, quite a few times (and people have done it for me.) But some wouldn't. And as I said, they don't have to. It's entirely up to them, and people shouldn't be judged for not wanting to do it. 
YANBU to ask though, but if they say they'd prefer not to, say 'sorry to have bothered you,' and smile, and say 'thank you anyway.'
It's your call. 