transfers in July ... too hot???

[Kelly in NY]

Are t-shirt transfers too hot to wear in July? I know this is short notice, we leave in 9 days...but we will be at WDW on Ds's 7th birthday and I was thinking of making shirts that say today's my birthday, its my brothers birthday etc. but then started thinking that maybe the transfers would be too hot. what to you all think?? if it is too hot, any other suggestions to make it noticeable that it is his actual birthday, not just celebrating?
Thanks for any advice or suggestions.