I'm guessing from the wording of your post that you are based in the USA. If so, then the law we need to apply is the Copyright Act 1976 as amended. Section 107
of that Act allows certain forms of fair use of copyright works, provided that the copy meets a majority of the four main criteria set out in the section. These are
(1) the purpose and character of the use, including whether such use is of a commercial nature or is for nonprofit educational purposes;
(2) the nature of the copyrighted work;
(3) the amount and substantiality of the portion used in relation to the copyrighted work as a whole; and
(4) the effect of the use upon the potential market for or value of the copyrighted work.
On the first point, a court could find that was in your favour due to the charitable nature of the operation. However the underlying purpose in both cases is the entertainment of children and so a skilled attorney acting for Spin Master might be able to convince the court that point one was a draw. Point two would likely go to the copyright owners a given that both depictions would be vehicles, just that one is a cartoon and the other is real life sized.
Point four would go in your favour as you are not marketing your vehicle so in no way affecting the market for normal Paw Patrol cartoons or their spinoff merchandise.
The decider is therefore the extent to which the original Paw Patrol artwork was copied. As far as I am aware the cartoons don't feature anything that looks like a hummer and so just painting the vehicle the same shade of blue wouldn't amount to any kind of infringement. But equally I don't suppose most kids would immediately connect a blue hummer with Paw Patrol and so you would also need to copy some of the decals. If that was the full extent of any copying then that would almost certainly be too insubstantial to amount to infringement.
So on balance, my hunch would be that this would be seen as fair use.
There may be a trade mark dimension to this. The original cartoon vehicle does not feature the words Paw Patrol or the brand logo and so if you decided to write that on your vehicle while it would not be copying the vehicle, it would likely infringe the trade marks which have been registered by Spin Master. So I would advise against that as, under the Lanham Act, there are no fair use exceptions similar to section 107. Theoretically there shouldn't be a trade mark issue as you would not be using the trade marks in the course of trade or commerce, but when a company decides to protect their intellectual property they tend to petition the court in a way which paints the full picture of the wrong being done to them (as they see it) even if they expect lose on a secondary point.
As it would very embarrassing for the police department if I am wrong about all of this, I suggest you should check with an intellectual property attorney who practises in your state and who would know how your local federal District court tends to decide on fair use, as it does vary across the country.